feat(meta_ads): operator-specified canonical conversion event (#342)

[hyoshi]

Summary

Follow-up to #340. The Meta conversion counter counts only a fixed deduped
generic set ({lead, purchase, complete_registration}), which under-counts
two account setups:

1. Custom-event advertisers — a real conversion that is a custom pixel
   event (offsite_conversion.custom.<id>) reports 0 conversions, so CPA
   can't compute and daily-check / budget decisions go wrong.
2. Component-only accounts — an account that emits only a component row
   (offsite_conversion.fb_pixel_lead) with no generic aggregate counts 0.

This lets the operator declare, per Meta ad account, exactly which
action_type(s) count as conversions, overriding the default.

Design

• Replacement semantics: the listed types are the complete conversion set
  — never summed on top of the generic set, so overlapping aliases can't
  double-count (preserves the bug(meta_ads): meta_row_conversions が action_type の部分一致でコンバージョンを過大計上（別名の重複＋ViewContent混入） #340 fix). Unset → built-in default (no change
  for accounts that report it). Empty → falls back to default (a cleared
  setting never zeroes everything).
• Storage: STATE.json platforms[<platform>].conversion_action_types
  (preserved across upsert_campaign / set_platform_metrics).
• Setter: new MCP tool mureo_state_set_conversion_events.
• Resolver load_conversion_action_types(account_id) reads the active
  workspace STATE.json via the runtime context (the same file the tool
  writes — agency / alternate-StateStore safe), tolerant of the act_
  prefix, and never raises (a malformed file → None, never breaks a live
  report). Threaded into all five live Meta conversion counters
  (meta_row_conversions, _extract_cv, _insights breakdown,
  _budget_efficiency, _live_clients aggregate/per-campaign,
  _summarise_meta_performance).
• Skill: _mureo-meta-ads gains a "conversion definition" section telling
  the agent to look up the account's real action_type labels (via
  meta_ads_insights_report / _breakdown), confirm with the operator, then
  set the exact string(s) — so custom slugs aren't mistyped. No configure-UI
  change (operator-via-agent flow, per design discussion).

Test plan

• count_conversions_from_actions override (replace / empty-fallback /
  component-only).
• set_conversion_action_types + load_conversion_action_types round-trip,
  clear, account mismatch, never-raises on malformed/non-object JSON,
  act_ prefix tolerance, preservation across upsert/metrics, parse↔render.
• MCP tool: set/clear, input validation, read/write path agreement via
  the runtime context.
• ruff + black clean on mureo/; mypy --strict clean on changed
  modules; 1336 relevant tests pass (only the known installed-plugin
  env-gating noise excluded). Tool-count assertions + docs updated (188 → 189).
• Reviewed by python-reviewer (adversarial); fixed two HIGH (read/write
  path mismatch, never-raises contract) + the act_ footgun + a str char-split
  edge — each with a regression test.

Known follow-up (out of scope)

The Protocol adapter's daily_report path (mureo/adapters/meta_ads/mappers.py)
uses its own hardcoded conversion set and is not yet covered by the override.

Closes #342

https://claude.ai/code/session_011rAu94b1o1xWYZhARk1VmL