feat: side-call context visibility (T3) — plan titles + side_call_usage events

[jkyberneees]

Summary

• B3 — step titles in side-call payloads: sideCallPlanPrefix emits 's2=in_progress - Title' (normalized, notes excluded). Compaction/budget payloads may be the only surviving context after a trim — bare IDs tell the summarizer nothing. Legacy anti-leak pins updated: titles required, notes still forbidden.
• Measurement — side_call_usage events: recordSideCallUsage(kind, res) emits a typed runtime event (kind, input/output/cache tokens) plus internal signal, so --events-jsonl can quantify compaction vs progress_summary cost — previously invisible inside run totals. Side calls still bypass lastPromptTokens.

Review-found fixes (3-judge panel)

• 'main_partial' kind for failed main-path partials (was 'interrupted_partial' — mislabeled side-call cost)
• milestone-map/title-separator hygiene (';' → ',' in titles)
• docs/EXTENSIONS.md event enumeration + data table updated; additive-safe (ignore-unknown-types rule, no version bump)

Test plan

• RED-first: internal/loop/sidecall_visibility_test.go
• internal/loop + internal/events green under -race; full cmd/odek green under -race (×2)
• 3 adversarial judges; all findings fixed or accepted-as-documented