feat: add pre-recon cache for incremental analysis

[jvinolas]

Disclaimer

We are using it to test the same code that evolves with new features and bugfixes, so we found interesting to avoid the long pre-recon phase and just update with diff. I used claude and build and we are using it, but feel free to discard it and instead take this as an improvement request issue if you prefer.

Summary

• Add pre-recon cache service that detects source code changes via git
  commit tracking
• When source is unchanged, skip pre-recon entirely; when changed, run a
  lightweight delta agent that updates the previous analysis based on the
  git diff
• Graceful fallback to full pre-recon when cache is missing, corrupt, or
  diff generation fails

Changes

• src/services/prerecon-cache.ts — Cache service with
  checkPrereconCache() and savePrereconCache()
• src/session-manager.ts — PRE_RECON_DELTA_AGENT definition (separate
  from ALL_AGENTS to avoid breaking resume)
• src/temporal/activities.ts — Cache check, save, and delta agent
  activities
• src/temporal/workflows.ts — Phase 1 cache-aware logic (skip / delta /
  full)
• src/services/agent-execution.ts — Optional AgentDefinition override
  for delta execution
• prompts/pre-recon-delta.txt — Delta agent prompt template

How it works

1. Before pre-recon, checkPrereconCacheActivity reads
   .prerecon-cache.json from the repo's deliverables directory
2. Compares the cached commit hash against current HEAD:
   • Same commit → skip pre-recon entirely
   • Different commit → write git diff to disk, run delta agent with
     previous analysis + diff context
   • No cache / invalid → run full pre-recon
3. After successful analysis, savePrereconCacheActivity writes the
   source commit hash (captured before agent execution to avoid checkpoint
   commit interference)

Test plan

• Run scan on a repo with no cache → full pre-recon runs, cache saved
• Re-run same scan without code changes → pre-recon skipped
• Pull new commits, re-run → delta agent runs with diff context
• Corrupt/delete cache file → graceful fallback to full pre-recon
• Verify resume still works correctly (pre-recon-delta not in
  ALL_AGENTS)