docs: define supervised agent-opened session contract

[wibus-wee]

Related issue

Refs #529

Context: #528, #531, and #569.

Problem / pressure

Agent-opened independent Sessions currently fragment one delegated workflow across ordinary rows, unread states, notifications, and permission routes. The product contract also needs to preserve the containment-only archive/delete behavior established by #569 instead of turning creation provenance into storage ownership.

Summary

• Add a draft Spec that defines worker-default supervision and explicit handoff in observable, storage-neutral terms.
• Cover roster/status/results, attention routing, permission escalation, settle, drill-in, archive/delete, phone creation, and existing openedBy* compatibility with acceptance scenarios.
• Record migration, acknowledgement, active-dismiss, lifecycle fallback, post-create handoff, and later-work routing as human decisions in a proposed Agent Note.
• Link the owning Session relation Spec to the new contract without describing proposed behavior as implemented.

Visual explanation

Human -> supervisor Session A -> worker B
                              `-> worker C
       -> explicit handoff Session D (independent peer)

supervision: roster + result + attention + acknowledgement
containment: workspace + archive/restore/delete target selection
provenance: opener identity + reverse navigation

Before / after

Before	After
Every independent agent-opened Session is presented as a peer, and openedBy* does not distinguish delegated work from handoff.	New agent-opened Sessions default to supervised workers; only explicit handoff creates a peer.
Worker unread, completion, and permission attention route directly to each worker.	One supervisor route owns aggregate attention while preserving exact per-worker drill-in and acknowledgement.
Archive expectations in #528 conflict with the containment-only lifecycle delivered by #569.	Supervision is explicitly non-containment; unresolved attention on supervisor archive/delete requires an approved product choice.

Test plan

• pnpm run docs check passes with no errors and no protected topics.
• Direct workspace formatting, lint, i18n, code-collab import, platform-boundary, and public-boundary checks pass.
• Adapter builds and direct workspace typechecks pass; root wrappers could not invoke missing corepack in this environment.
• pnpm test:scripts passes 27/27.
• Workspace tests under Node 26 pass broadly after an explicit local-storage test file; the components suite has two unrelated failures in app-store-review-prompt-hook.test.tsx and tasks-beta-gate.test.tsx (3464 passed, 2 failed). Electron tests were excluded by the repository CI test command.

Context handoff

Instructions for reviewing agents

• Review focus: Review specs/session-worker-supervision.md for routing and lifecycle consistency with specs/session-relations.md.
• Decisions to challenge: Challenge the recommended legacy classification, per-worker acknowledgement, terminal-only settle, and supervisor delete precondition.
• Plausible failures / evidence gaps: No storage shape or migration is proposed, and archive/delete attention fallback remains blocked on human approval.

Authoring context

• User goal / directives: Define the product contract for agent-opened Sessions as supervised workers unless explicitly handed off.
• Constraints / non-goals: Documentation only; do not change runtime behavior, storage, or containment lifecycle.
• Risk-bearing decisions: Attention ownership, mixed-client fallback, acknowledgement scope, and destructive lifecycle preconditions affect compatibility and recovery.
• Destructive or irreversible behavior: The draft forbids implicit worker cascade and requires approval before supervisor deletion can strand unresolved workers.
• Deliberately not done or tested: No implementation, migration, translation, or Electron test run is included in this product-contract phase.
• Unknowns / confidence: The direction is internally coherent, but the explicit decision table must be approved before implementation.