Add --atuin-db to load history from an atuin SQLite database

[Copilot]

Adds a --atuin-db <PATH> flag that, when set, loads history from an atuin SQLite database instead of parsing bash history. Implemented without the atuin crate, using the lightweight sqlite wrapper with bundled libsqlite3.

Changes

• CLI / Settings: new --atuin-db <PATH> flag plumbed through Settings::atuin_db_path.
• History source switch: in HistoryManager::new, when atuin_db_path is set, atuin entries replace the bash entries; --load-zsh-history still merges on top unchanged.
• Atuin reader (history.rs): opens the DB read-only and runs:

  SELECT timestamp, command FROM history
   WHERE deleted_at IS NULL
   ORDER BY timestamp ASC

  Atuin's nanosecond timestamps are converted to seconds. Missing file / open / decode errors degrade to an empty history with a logged warning, matching the existing zsh-history error UX.
• Dependency: sqlite = "0.37" with the bundled feature — no system libsqlite3 required, no atuin transitive deps. Advisory DB checked: clean.
• Tests: unit tests covering the happy path (in-memory atuin-shaped DB written to a temp file; verifies ordering, deleted_at filtering, and ns→s conversion) and the missing-file path.

--release libflyline.so size

	bytes	~MiB
before	4,177,544	3.98
after	5,619,728	5.36
Δ	+1,442,184	+1.38

Increase is essentially the statically-linked bundled libsqlite3.