Delegate cost reporting: use canonical cumulative format

[gnovak]

Per the design discussion in chat: delegate should look like calling the individual steps in sequence. Per-step cost on Stage 1, 3, 3a, 3c already worked. What was wrong was the trailing "Delegate Stages 1-3c Aggregate Cost" block.

Two problems with the old aggregate block

1. Bespoke label format — different from what individual /agent-* invocations emit
2. Used per-step cost format internally — bold `$X.XX` is the marker that the cumulative-cost scanner picks up as a prior step. The aggregate would have been double-counted on any subsequent /agent-* on the same issue → guaranteed quadratic growth

Fix

Each per-step cost block in delegate now carries a canonical cumulative table appended (via `compute_cumulative_table` from `lib/cumulative_cost.py` — the same code individual invocations use). Cumulative is empty on Stage 1 (no prior cost markers); from Stage 3 onward it includes all prior delegate stages. The cumulative on the last reported stage IS the running total — no aggregate block needed.

Council reviewer comments (Stage 2, 3b) intentionally remain cost-table-less per the existing decision: their costs are small and reporting them would just be noise. Their costs won't appear in the cumulative, which matches the "what shows in the cumulative is exactly what was reported per-step" rule.

No new cost-reporting code paths — this just routes delegate through the existing per-step + cumulative formatting that resolve/design/review already use. The `_build_stage_cost_block` helper added in `workshop.py` is a thin wrapper that calls `_build_cost_table` + `compute_cumulative_table`.

`run_delegate` gains two parameters (`github_repo`, `issue_number`) so the cumulative scanner can fetch prior comments. The workflow passes them in from the same env vars it already uses.

Test plan

• 662 unit tests pass
• Next `/agent-delegate` run should show: Stage 1 with per-step cost only; Stages 3/3a/3c each with per-step cost + Cumulative (N steps) block; no separate Aggregate Cost comment

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