fix(operations): per-project git sync + completion receipts in brief (S2, S3)

Brief.Git was a single GitSync read from ORCHESTRA_DATA (never a git
checkout), and completions were counted but discarded their report_ref/
receipt. Brief.Git is now keyed by project ID and built from each
project's real repo; GitSync gained Ahead/Behind vs upstream; Brief now
carries Receipts pulled from each TaskCompleted payload.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT

AUDIT.md

@@ -840,6 +840,33 @@ by `TestFanoutContinuesAfterSendError` (`internal/delivery/delivery_test.go`
 sender both receive the event, the cursor still advances, and `Run` only
 exits on context cancellation, never on the send error.
 
+### S2, S3 — closed, 2026-07-27
+
+`Brief.Git` was a single `GitSync` read from `ORCHESTRA_DATA` (the event-log
+directory, never a git checkout, so it always reported `"git unavailable"`) —
+S2 named this and asked for per-project state from the actual worktrees.
+`Brief.Git` is now `map[string]GitSync` keyed by project ID; `/v1/brief`
+(`cmd/orchestra/main.go`) builds it from `registry.Project.Repo` for every
+project that sets one, falling back to a single `"default"` entry keyed off
+`ORCHESTRA_REPO` for single-repo deployments that predate per-project repos.
+`operations.GitState` also gained `Ahead`/`Behind` (via
+`git rev-list --left-range --count @{u}...HEAD`) so "what pushed" and "what
+workpc still needs to pull" (§7.4's own phrasing) are both answerable, not
+just branch/HEAD/dirty.
+
+S3: `BuildBrief` counted `TaskCompleted` but discarded the payload, so the
+brief never carried the `report_ref`/receipt proofs §7.4 names as the reason
+completions are surfaced at all ("the receipts for every completion"). Added
+`operations.CompletionReceipt{TaskID, ReportRef, Receipt}` and
+`Brief.Receipts []CompletionReceipt`, populated straight from each
+`TaskCompleted` event's existing payload (no new event fields needed —
+`domain.ValidatePayload` already requires both on every `TaskCompleted`).
+Covered by an updated `TestBuildBrief` (`internal/operations/operations_test.go`)
+asserting a completion's `report_ref`/`receipt` and a project's `GitSync`
+both come through in the brief.
+
+`go build ./...`, `go vet ./...`, `go test ./...` all pass.
+
 ### Design consequences (not yet implemented)
 
 1. **Percentages are a level, not a delta.**