Overseer: namespace-scope the event/inbox substrate (multi-tenant)

[heavygee]

Problem

Codex (and AGENTS.md) correctly flag that Overseer read/write tools resolve sessions and events without threading the authenticated CLI namespace. On a multi-CLI_API_TOKEN hub, an operator authenticated to namespace A can target another namespace's session by full ID / unique prefix (relay, state reads, etc.).

This shows up as a repeated P1 on every Overseer PR (#98–#106 stack) because system_events / inbox rows have no namespace column — there is nothing honest to filter on today.

Goal

One substrate epic, not per-PR cosmetic namespace args:

1. Persist namespace (or equivalent tenant key) on Overseer events + inbox items at write time.
2. Thread authenticated namespace from hub routes into OverseerEntity accessors.
3. Fail closed: refuse cross-namespace session resolve / relay / disposition.
4. Migration / backfill plan for existing rows (default namespace vs unknown).

Non-goals

• Fake per-PR filters that look namespaced but still scan global tables.
• Blocking the current converse/relay dogfood stack on this alone (single-tenant operator hub is the current dogfood).

Origin

Repeated Codex P1 across Overseer stack PRs; latest callout on #104 overseerEntity relay target resolution.

Kill criteria

Ship is done when a session id that exists only in namespace B cannot be resolved or relayed by a namespace-A converse token, with tests.

[heavygee]

Interim kill criterion shipped (session-scoped fail-closed)

Not the full substrate yet (no namespace column / backfill on events+inbox — that remains this epic).

What landed on the Overseer stack tip (0ef4d512b, #106):

• SyncEngine.getOverseer(namespace) — per-namespace OverseerEntity with resolveSessionAccess / getSessionsByNamespace
• Routes pass c.get('namespace') into Overseer + active-brain KV
• Relay uses allowFreshSpawn: false (no never-started stub spawn via ping_session)
• Cross-ns resolve/relay/disposition refuse; test: OverseerEntity namespace isolation (#107 kill criterion)

Still open on this issue: persist namespace at write time + migration/backfill for rows without relatedSessionId honesty.