refs(#2264): Phase 4 — replace Session.messages: Vec<Message> with AppendLog

[encyc]

Refs #2264 (Phase 4 — AppendLog backing store).

Summary

Replaces Session.messages: Vec<Message> with AppendLog, the
append-only wrapper introduced in Phase 1. AppendLog now implements
Deref<Target=Vec<Message>> + DerefMut + From/Into, making
it a transparent drop-in — all existing .len(), .iter(),
.push(), .remove(), .clone() calls work without change.

AppendLog changes

• Add Deref/DerefMut for transparent Vec access
• Add From<Vec<Message>> and Into<Vec<Message>> for conversions
• Add into_inner() for explicit unwrapping
• Remove explicit iter()/len()/is_empty()/as_slice() — Deref
  provides std::slice::Iter (not opaque impl Iterator)

Files

File	Change
prompt_zones.rs	+28/−25: Deref + From impls
session.rs	+2/−2: messages: AppendLog
engine.rs	+8/−8: .into() on assignments
turn_loop.rs	+2/−2: .into() + .into_inner()
capacity_flow.rs	+1/−1: .into()
tests.rs	+2/−2: &* deref for assertions

Testing

• 37 prompt_zones + prefix_cache tests pass
• cargo check: clean (6 pre-existing schema_migration warnings)

Greptile Summary

This PR wires AppendLog (from Phase 1) as the backing store for Session.messages, replacing the raw Vec<Message>. The conversion is kept transparent by adding Deref<Target=Vec<Message>>, DerefMut, From/Into, and into_inner(), so all existing call sites continue to compile with minimal mechanical changes.

• AppendLog in prompt_zones.rs gains Deref, DerefMut, and the From/Into pair; the redundant explicit methods (len, is_empty, iter, as_slice) are removed in favour of slice auto-deref.
• Session.messages in session.rs is retyped to AppendLog; eight assignment sites in engine.rs, capacity_flow.rs, and turn_loop.rs add .into() to convert Vec<Message> results into AppendLog.
• Two test assertions are updated to use &* to explicitly deref AppendLog to &Vec<Message> for comparison.

Confidence Score: 4/5

Safe to merge — the mechanical conversions are all correct and cargo check is clean. The only concern is a design gap in prompt_zones.rs that future contributors should be aware of.

All Vec to AppendLog assignment sites are converted correctly, type inference via From/Into is sound, and the test updates are accurate. The one open question is DerefMut: it fully exposes the inner Vec for arbitrary mutation (remove, truncate, clear, last_mut), so the append-only contract is documented but not enforced at the type level. Existing callers already depend on those mutations, so no behaviour changes with this PR, but the abstraction is now leaky and could mislead future contributors into thinking the type guards against removals or rewrites.

crates/tui/src/prompt_zones.rs — the DerefMut implementation and its interaction with the append-only doc-comment.

Important Files Changed

Filename	Overview
crates/tui/src/prompt_zones.rs	Adds Deref/DerefMut, From/Into, and into_inner() to AppendLog; removes the explicit iter/len/is_empty/as_slice methods. DerefMut fully exposes the inner Vec for arbitrary mutation, undermining the append-only invariant the type is named and documented for.
crates/tui/src/core/session.rs	Changes Session.messages field from Vec to AppendLog and initialises it with AppendLog::new(); minimal and correct change.
crates/tui/src/core/engine.rs	Updates eight assignment sites from direct Vec assignment to .into() conversions; all existing .remove(), .truncate(), .len(), and .iter() calls continue to work via DerefMut/Deref.
crates/tui/src/core/engine/turn_loop.rs	Converts one assignment site to .into() and changes messages_with_turn_metadata() to use clone().into_inner(); the latter allocates an unnecessary intermediate AppendLog.
crates/tui/src/core/engine/capacity_flow.rs	Single assignment converted to .into(); no other changes.
crates/tui/src/core/engine/tests.rs	Updates two assertions to use &* deref to compare Vec with &*AppendLog; change is correct.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["result.messages: Vec<Message>"] -->|".into()"| B["AppendLog"]
    B -->|"Deref"| C["&Vec<Message> (read-only callers)"]
    B -->|"DerefMut"| D["&mut Vec<Message> (remove / truncate / clear / last_mut)"]
    B -->|".clone().into() or .clone().into_inner()"| E["Vec<Message> (SessionUpdated / API calls)"]
    B -->|"into_inner()"| F["Vec<Message> (consumed)"]
    G["Session.messages: AppendLog"] --- B

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Greptile also left 2 inline comments on this PR.

[gemini-code-assist]

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[github-actions]

Thanks @encyc for taking the time to contribute.

This repository is currently observing a maintainer-managed contribution gate in dry-run mode, so this pull request is staying open. When enforcement is enabled, pull requests from contributors who are not listed in .github/APPROVED_CONTRIBUTORS will be closed automatically.

Please read CONTRIBUTING.md for the expected contribution shape. A maintainer can grant PR access by commenting /lgtm on a pull request.

[greptile-apps]

DerefMut exposes the full Vec<Message> mutation API, silently defeating the append-only contract. Callers in the current codebase already reach through it to call .remove(0) (engine.rs:1821), .truncate(idx) (engine.rs:1139), .clear() (capacity_flow.rs:690), last_mut() (turn_loop.rs:1127), and .extend_from_slice() (session_manager.rs:745). The type's name and doc-comment still say "append-only", but there is no compile-time barrier left to prevent arbitrary removals or rewrites — any future call site can silently violate the invariant without the compiler warning. If the intent is for AppendLog to enforce the constraint, DerefMut should be removed and explicit methods added for each needed mutation. If unrestricted mutation is acceptable, the "append-only" branding in the doc-comment should be updated to avoid misleading future readers.

Suggested change

	impl std::ops::DerefMut for AppendLog {
	fn deref_mut(&mut self) -> &mut Self::Target {
	&mut self.messages
	}
	}
	// NOTE: DerefMut is intentionally omitted. Exposing &mut Vec<Message>
	// would allow callers to call .remove() / .truncate() / .clear() etc.,
	// defeating the append-only invariant. Add explicit methods for each
	// permitted mutation instead (e.g. `truncate`, `clear`, `remove_first`).
	//
	// impl std::ops::DerefMut for AppendLog { … } ← deliberately absent

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[greptile-apps]

clone().into_inner() allocates a full AppendLog clone just to immediately unwrap it into Vec<Message>. Since Deref<Target = Vec<Message>> is now implemented, the inner Vec can be cloned directly without constructing the intermediate AppendLog.

Suggested change

	self.session.messages.clone().into_inner()
	(*self.session.messages).clone()