feat(costs): redesign session cost tracking with wisps and daily digests (#292)

* feat(costs): redesign session cost tracking with wisps and daily digests

Implement the wisp-based cost tracking architecture per gt-cm900:

- gt costs record now creates ephemeral wisps (not exported to JSONL)
  to avoid log-in-database pollution with O(sessions/day) events

- gt costs digest aggregates yesterday's session wisps into a single
  permanent "Cost Report YYYY-MM-DD" bead for audit purposes

- gt costs query updated: --today queries wisps, --week queries
  digest beads + today's wisps

- gt costs migrate closes legacy open session.ended beads

- Deacon patrol formula updated with costs-digest step

The new architecture:
  Session ends -> Wisp (fast, N/day) -> Patrol digest -> Bead (1/day)

This preserves audit trail while keeping issues.jsonl clean.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* chore: sync canonical formula with embedded copy

Update .beads/formulas/ with the costs-digest step added to
mol-deacon-patrol.formula.toml. The go:generate copies from
.beads/formulas/ to internal/formula/formulas/.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/formulas/mol-deacon-patrol.formula.toml

@@ -563,10 +563,48 @@ Skip dispatch - system is healthy.
 
 **Exit criteria:** Session GC dispatched to dog (if needed)."""
 
+[[steps]]
+id = "costs-digest"
+title = "Aggregate daily costs"
+needs = ["session-gc"]
+description = """
+**DAILY DIGEST** - Aggregate yesterday's session cost wisps.
+
+Session costs are recorded as ephemeral wisps (not exported to JSONL) to avoid
+log-in-database pollution. This step aggregates them into a permanent daily
+"Cost Report YYYY-MM-DD" bead for audit purposes.
+
+**Step 1: Check if digest is needed**
+```bash
+# Preview yesterday's costs (dry run)
+gt costs digest --yesterday --dry-run
+```
+
+If output shows "No session cost wisps found", skip to Step 3.
+
+**Step 2: Create the digest**
+```bash
+gt costs digest --yesterday
+```
+
+This:
+- Queries all session.ended wisps from yesterday
+- Creates a single "Cost Report YYYY-MM-DD" bead with aggregated data
+- Deletes the source wisps
+
+**Step 3: Verify**
+The digest appears in `gt costs --week` queries.
+Daily digests preserve audit trail without per-session pollution.
+
+**Timing**: Run once per morning patrol cycle. The --yesterday flag ensures
+we don't try to digest today's incomplete data.
+
+**Exit criteria:** Yesterday's costs digested (or no wisps to digest)."""
+
 [[steps]]
 id = "log-maintenance"
 title = "Rotate logs and prune state"
-needs = ["session-gc"]
+needs = ["costs-digest"]
 description = """
 Maintain daemon logs and state files.