feat(cost-diagnostics): content-free tool census and compaction counter

[philmerrell]

Summary

PR 3 of 3. The one signal the admin session profile could not derive from existing rows: which tools a conversation called, how often, and how often they failed — plus how many times compaction actually fired. Recorded out of band at turn end, never in the conversation, so the cacheable prefix is untouched. Independent of the other two PRs (the API tolerates the attributes' absence).

• ToolCensusHook tallies tool name → {calls, errors} per model call using the same cycle counter AgentStatusHook uses; non-drained, per-turn only. The coordinator reads each call's tally at turn end and attaches it to that call's C# row as toolCalls (the turnAgentId mechanism).
• _bump_session_aggregates ADDs toolCallCount / toolErrorCount in the same UpdateItem as totalCost.
• _save_compaction_state(record_event=True) ADDs a monotonic compactionCount when a checkpoint is actually cut — the persisted compaction map is last-write-wins and cannot count occurrences; a top-level ADD never moves backwards even with two Agents on one session.
• COST_DIAGNOSTICS_ENABLED — default on, =false kill switch (unset/empty ⇒ on). With it off nothing is written, so the profile reads "not tracked" rather than an honest-looking 0.

Deploy notes

🏗️ backend.yml only. Additive attributes on rows the turn already writes — no table, no index, no backfill. Sessions that predate this read "not tracked" in the admin profile. Forks need to do nothing; the census appears as data accrues.

Verification

• uv run python -m pytest tests/ — full suite green (16 new: hook attribution/reset/non-drained reads/kill switch, moto persistence on C# + S#, coordinator attach seam, compaction counter + kill switch).

🤖 Generated with Claude Code