refactor(session): extract SessionState into injectable class

[aaight]

Summary

• Extracts the mutable module-level let sessionState = {...} singleton in src/gadgets/sessionState.ts into an injectable SessionState class
• Adds createSessionState() factory for creating isolated instances (enables parallel test isolation)
• Adds setDefaultSessionState() for dependency injection of custom instances
• Preserves all existing exported functions (getBaseBranch, getProjectId, recordPRCreation, etc.) as thin wrappers around a _defaultInstance, so all 17 consumers work without any import changes
• Updates tests/unit/gadgets/sessionState.test.ts to cover both the class interface and all backward-compatible module-level wrappers

Closes: https://trello.com/c/69c1983cea2a43b1e42d6391

Test plan

• All 52 sessionState tests pass (24 original + 28 new class/factory/DI tests)
• Full unit test suite passes (6577 tests, 0 regressions)
• TypeScript type check passes (0 errors)
• Biome lint passes (0 errors)

🤖 Generated with Claude Code

🕵️ claude-code · claude-sonnet-4-6 · run details

[codecov]

Codecov Report

❌ Patch coverage is 99.09091% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/gadgets/sessionState.ts	99.09%	1 Missing ⚠️

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[nhopeatall]

Summary

LGTM — clean, behavior-preserving refactoring that extracts mutable module-level state into an injectable SessionState class while keeping full backward compatibility via thin wrapper functions.

Verified:

• All 13 previously exported module-level functions are preserved as wrappers delegating to _defaultInstance
• The SessionState class correctly replicates all original behavior (init reset, shallow copy in getSessionState(), error recovery in deleteInitialComment)
• SessionStateData is correctly kept as an unexported interface (consumers access properties structurally)
• getSessionState() returns a shallow copy via spread ({ ...this.state }), matching the original behavior — the hooks object is never mutated post-init so shallow copy is safe
• Factory (createSessionState) and DI setter (setDefaultSessionState) provide proper test isolation
• Tests cover class interface, factory isolation, DI injection, and all backward-compatible wrappers
• All 7 CI checks pass

🕵️ claude-code · claude-opus-4-6 · run details