docs(omo-codex): document session_start telemetry source

Spell out that omo_codex_daily_active now fires from two sources -
install_completed (CLI installer) and session_start (Codex plugin
SessionStart hook) - so readers understand DAU/WAU/MAU reflects real
Codex usage and not just install events. Opt-out env vars are unchanged.

- README.md (root + packages/omo-codex/): both reasons listed.
- docs/legal/privacy-policy.md: discloses session_start as a second
  trigger for the same event, same opt-out posture.
- docs/reference/cli.md: telemetry section now describes both PostHog
  streams (omo_daily_active vs omo_codex_daily_active) and both Codex
  reasons.
- CHANGELOG.md: adds the plugin-runtime wiring under Unreleased.

docs/reference/cli.md

@@ -72,7 +72,12 @@ When using the `lazycodex` bin alias, `install` defaults to `--codex=yes`.
 
 ### Telemetry and opt-out
 
-Anonymous telemetry uses PostHog with a hashed installation identifier.
+Anonymous telemetry uses PostHog with a hashed installation identifier. Two streams exist:
+
+- `omo_daily_active`: fired by the main plugin and `oh-my-openagent run`.
+- `omo_codex_daily_active`: fired by `omo install --codex=yes` (`reason: "install_completed"`) and by the Codex plugin's `SessionStart` hook on every Codex session (`reason: "session_start"`). Both sources share the same UTC-day deduplication, so daily/weekly/monthly active counts reflect real Codex usage, not just install events.
+
+Opt-out env vars:
 
 - Global opt-out for oh-my-openagent and omo-codex: `OMO_SEND_ANONYMOUS_TELEMETRY=0` or `OMO_DISABLE_POSTHOG=1`
 - Codex-only opt-out for `omo_codex_daily_active`: `OMO_CODEX_SEND_ANONYMOUS_TELEMETRY=0` or `OMO_CODEX_DISABLE_POSTHOG=1`