docs(policy): permanently codify design/execution model split (model delegation policy)

[Yuu6798]

Completion Summary: dev-workflow-rules (design / execution model split)

Phase

Repository operating policy (CLAUDE.md / docs). Port of the ecosystem-wide dev-workflow rule (design-model delegation discipline) into the code domain. Product code untouched.

What Changed

• CLAUDE.md ## Workflow gains a Model split section (always-loaded operative rule): Fable (design model) is fixed to design judgment only — design, specification, review verdicts, brief drafting, phase planning. Implementation, execution, verification, and dogfooding are delegated to Opus/Sonnet executors. Route: Fable designs → Opus/Sonnet implements・executes・verifies → Fable judges the evidence.
• Direct tool calls by Fable are allowed only when the whole operation is 1–2 calls AND every return payload is lightweight. Review-thread fetching and reply posting are always delegated regardless of call count — the measured token burner is GitHub tool return payloads (full-thread refetch, echoed diffs), not the fix work itself. When in doubt, delegate.
• New docs/model_delegation_policy.md (ACTIVE, 101 lines): full policy — measured rationale, role table, main route with distilled-summary handback, direct-call criteria with examples, always-delegate list, relationship to the AGENTS.md Codex handoff (unchanged; Claude-exception route now runs implementation through executor subagents), user override, scope-guard non-goal, enforcement notes.
• Line budget: to respect the 300-line CLAUDE.md cap, detail is offloaded to the new doc; a duplicated stale-memory sentence (Tier D) was removed and the Tech Stack list compressed to prose — zero rules removed. CLAUDE.md lands at 297/300.
• Design Documents table (+1 ACTIVE row) and README documentation list updated per the CLAUDE.md doc-registration rule.

Acceptance Criteria Status

• Operative rule lives in the always-loaded CLAUDE.md ## Workflow — Model split section present with route + direct-call criteria
• Thread fetch / reply posting marked always-delegate regardless of call count — encoded in both CLAUDE.md and the policy doc §5
• Full policy externalized to docs/model_delegation_policy.md and registered in the Design Documents table + README — both cross-references verified
• CLAUDE.md ≤ 300-line discipline cap holds — 297 lines, test_claude_md_line_cap green
• AGENTS.md Codex handoff unchanged — no edits to AGENTS.md; relationship pinned in policy doc §6

Tests

• Added: none (agent-behavior policy is not structurally enforceable by pytest; enforcement = always-loaded CLAUDE.md rule, see policy doc §8)
• Result: python -m pytest tests/discipline/ -q --no-cov → 36 passed; ruff check . → clean

Dogfooding

• Status: performed
• Commands: verification (discipline suite + ruff + cross-reference sanity) was delegated to a Sonnet executor subagent per the new rule itself; the design model received only the distilled summary
• Result: 36 passed / ruff clean / all cross-references resolve — first live pass of the Fable→executor→Fable route this PR codifies

Files Changed

CLAUDE.md                        | 43 +++++++++++++++--------
README.md                        |  1 +
docs/model_delegation_policy.md  | 101 ++++++++++++++++++++++++++++++ (new)
3 files changed, 126 insertions(+), 20 deletions(-)

Deviations from Brief

None. (Provenance note: the 7-round-review token measurement originates from an ecosystem sibling-repo session, so the policy doc cites it as an ecosystem measurement rather than a PR number in this repo, avoiding a false cross-reference to this repo's #149.)

Open Questions / Deferred

• Leading indicator (direct GitHub-call count per Fable session / tokens per review round) is observational for now; revisit at wrap-up whether any part can be test-encoded.

Next Handoff

Review the Model split wording in CLAUDE.md ## Workflow and the always-delegate list in docs/model_delegation_policy.md §5; merge when satisfied.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CmnG4powRuRvD37unq7i1m

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