Promote auto-compaction config to typed per-org/per-user settings

[haasonsaas]

Context

We already have auto-compaction working in src/agent/auto-compaction.ts (header comment: "Inspired by Claude Code's auto-compaction"). Today the knobs are process env vars:

• MAESTRO_AUTOCOMPACT_PCT (default 85)
• MAESTRO_AUTOCOMPACT_ENABLED (default true)
• MAESTRO_AUTOCOMPACT_MIN_MESSAGES (default 10)

Sourcegraph Amp's "Neo" rebuild ships the same threshold as a typed user setting (internal.compactionThresholdPercent) validated by zod, alongside internal.model, internal.oracleReasoningEffort, openai.speed, etc. The settings schema itself becomes an audit-able artifact: "what knobs even exist." See ampcode.com/news/neo.

We have the storage already — db/schema.ts has users.settings: jsonb<UserSettings> and organizations.settings: jsonb<OrganizationSettings> columns sitting empty for this. We just don't have a typed catalog to go in them.

Scope

1. Add a MaestroSettingsSchema (TypeBox, alongside the existing packages/contracts/src/schemas.ts) covering at minimum:
   • internal.compactionThresholdPercent (number, 50–100)
   • internal.compactionEnabled (boolean)
   • internal.compactionMinMessages (number, ≥0)
   • internal.compactionKeepRecentTokens (number, ≥0) — the existing token-based cut point in src/agent/compaction.ts
2. Resolution order at runtime: org settings → user settings → env var (legacy fallback) → built-in default.
3. Wire resolution into getAutoCompactionConfig() so callers don't change shape.
4. Surface the schema through amp plugins show-docs-equivalent (we don't have one yet — could reuse the headless control plane handshake's ClientCapabilities field to advertise settings keys instead).
5. Audit hook: emit a settings_changed event with the (org, user, key, old, new, by) tuple when settings flip — mirrors SERVER_REQUEST_RESOLVED_BY_* attribution model.

Non-goals

• Not migrating all env vars — only auto-compaction in this slice. Other MAESTRO_* env vars become candidates in follow-up issues.
• Not building a settings UI. Settings are edited via API/CLI for now.

Acceptance criteria

• Setting internal.compactionThresholdPercent on an org JSON column changes behavior for new sessions of users in that org without restarting maestro.
• Env var still works for users who don't have org/user settings (back-compat).
• Schema is exported from packages/contracts and consumed by both the runtime and the platform's audit pipeline.
• One settings change emits exactly one audit event.

Why now

We already have the implementation, the proto event (CompactionMessage in proto/maestro/v1/headless.proto:458), and the DB columns. This is a wiring task, not a build task — the cost is low and the audit-grade payoff is real.