agent: event-based project-context reinjection (full/delta)

[jjcovert]

Summary

Implements event-based reinjection for Project Context files to reduce repeated prompt token usage on steady-state turns.

Behavior

• Full context injection on:
  • session start/reset
  • post-compaction boundary
  • model/provider handoff
• Delta context injection on:
  • steady-state turns

What changed

• Added run-level context injection controls (promptContextMode, promptContextReason, compactionCountBeforeRun)
• Added event-based mode selection in reply-run assembly
• Added full/delta Project Context rendering mode in system prompt builder
• Added session prompt report metadata (contextInjectionMode, contextInjectionReason, compactionCountAtBuild, fullPromptCompactionCount)

Validation

• corepack pnpm install --frozen-lockfile
• corepack pnpm vitest src/auto-reply/reply/agent-runner-utils.test.ts --run
• corepack pnpm vitest src/agents/sanitize-for-prompt.test.ts --run

Notes

Repo-wide tsc --noEmit currently reports unrelated pre-existing e2e typing failures in src/gateway/server.chat.gateway-server-chat-b.e2e.test.ts.