fix(copilot): read VS Code chatSessions so Copilot Chat users see cost (#555)

[ozymandiashh]

Summary

Fixes #555. VS Code GitHub Copilot Chat users with no OTel agent-traces.db and no ~/.copilot/session-state/ saw $0.00 cost (calls counted, tokens all zero). Root cause: the provider never read VS Code's core chat persistence, which is the only on-disk source carrying their token counts:

• workspaceStorage/<hash>/chatSessions/*.jsonl
• globalStorage/emptyWindowChatSessions/*.jsonl

The existing sources (legacy session-state, OTel SQLite, legacy GitHub.copilot-chat/transcripts) either don't exist for these users or carry no tokens, so cost collapsed to zero.

What changed

• New fourth source in src/providers/copilot.ts. These files are a VS Code delta-journal, not flat records: a kind:0 snapshot followed by kind:1 path-set edits ({"k":[...],"v":...}) and kind:2 array appends ({"k":["requests"],"v":[...]}). The parser replays the journal to reconstruct requests[], then reads result.metadata.promptTokens / outputTokens (fallback completionTokens) and prices on result.metadata.resolvedModel.
• Prototype-pollution-safe replay: __proto__ / prototype / constructor path segments are rejected and intermediate maps use Object.create(null) (clears the repo's no-bracket-assign-hot-paths semgrep rule).
• No double counting: prefer OTel (skip chatSessions discovery when an OTel source is present) and skip a workspace's legacy transcripts when that same workspace has chatSessions.
• New env overrides CODEBURN_COPILOT_GLOBAL_STORAGE_DIR (+ existing CODEBURN_COPILOT_WS_STORAGE_DIR) keep discovery testable.
• Docs updated in docs/providers/copilot.md.

Testing

• npx vitest run tests/providers/copilot.test.ts -> 43/43. New chatSessions block covers: the reporter's real request object (promptTokens 32543 / outputTokens 60 -> non-zero cost), empty sessions, emptyWindow discovery, append-then-edit replay, requestId dedup, prototype-pollution paths, the transcript-skip, and the OTel-prefer skip.
• End-to-end against the reporter's exact After updating to 0.9.14 it doesn't show any cost #555 data:

  call: project=myproject model=claude-sonnet-4-6 in=32543 out=60 cost=$0.098529
  TOTAL  calls=1  in=32543  out=60  cost=$0.098529   (was 0 / $0.00)
  

• npx tsc --noEmit clean; semgrep hot-path rule -> 0 findings.
• Full suite: the only failing test is the pre-existing overview.test.ts thousands-separator case, which fails on non-US locales independent of this change (reproduces on main).

Notes

• This journal has no cache-token field, so cache read/write show as 0 for Copilot Chat unless the OTel store is enabled (documented).