refactor(agents): remove dead legacy agent execution path (~2,100 lines)

[aaight]

Summary

Removes ~2,100 lines of dead legacy agent execution code that was left over after the codebase migrated to the unified backend adapter architecture (backends/adapter.ts → agent-profiles.ts).

• Deletes 4 dead per-agent files: respond-to-ci.ts, review.ts, respond-to-pr-comment.ts, respond-to-review.ts
• Deletes 5 dead shared infrastructure files: lifecycle.ts, githubAgent.ts, prResponseAgent.ts, workItemBuilder.ts, workItemContext.ts
• Deletes dead base.ts (executeAgent, AgentContext, AgentRunner were all unreachable)
• Cleans up agents/index.ts — removes re-export of deleted executeAgent/AgentContext/AgentRunner
• Removes dead helpers from syntheticCalls.ts (injectDirectoryListing, injectContextFiles, injectSquintContext) — only injectSyntheticCall remains (used by live backends/llmist/index.ts)
• Fixes FileLogger type imports in cleanup.ts and runTracking.ts (were importing from deleted lifecycle.ts, now import from live executionPipeline.ts)
• Deletes 3 test files that exclusively tested dead code; trims syntheticCalls.test.ts to match remaining surface

Test plan

• npm run typecheck — zero errors
• npm test -- --run — 187 test files, 3190 tests, all passing
• npm run lint — zero errors (1 pre-existing complexity warning unrelated to this change)

🤖 Generated with Claude Code

Card: https://trello.com/c/c4d69Z1c/115-find-top-candidate-for-refactoring-and-plan-clean-refactoring-of-it-look-for-god-classes-modules-functions-files-code-duplicatio

[nhopeatall]

🤖 Just a sec, looking into that refactor for the legacy agent execution path

[nhopeatall]

LGTM — clean dead code removal. All deleted symbols (executeAgent, AgentContext, AgentRunner, executeAgentLifecycle, injectDirectoryListing, injectContextFiles, injectSquintContext, fetchImplementationSteps, and the four per-agent execution files) have zero remaining references in the codebase. The FileLogger type import fix in cleanup.ts and runTracking.ts correctly points to the canonical export in executionPipeline.ts. The sole surviving export from syntheticCalls.ts (injectSyntheticCall) is confirmed used by the live backends/llmist/index.ts. CI passes.