Tasks: the harness is a per-work-item choice at `phase-selection`

Each task names the requirement it satisfies and the testing-plan row that proves it.

Task list

[x] A1: hosting and vocabulary. hosts_sessions, ADAPTER_TYPES, hosting_harnesses, offered_harnesses, default_harness, the non-hosting-default finding. Req: R1.2, R4 · Test: T1 · Depends: —
[x] A2: state. harness on WorkItemState, the repository-attribute registry and the runtime's freeze loop. Req: R1.6, R5.2 · Test: T5 · Depends: —
[x] A3: gate. Bootstrap seeds offeredHarnesses and the shared default; the selection hook renders, parses, confirms and freezes the harness. Req: R1, R2, R5.1 · Test: T2, T3, T4 · Depends: A1, A2
[x] A4: dispatcher. _harness_for, _default_harness, spawn on the resolved harness. Req: R3, R4.1 · Test: T6 · Depends: A1, A2
[x] A5: Slack mirror. harness- prefix and note. Req: R1 · Test: T7 · Depends: A3
[x] A6: docs. Config schema/template descriptions, interactive-sessions.md, process-graph.md, the skill's phase-selection text. Req: NFR · Test: T10 · Depends: A3, A4
[x] A7: verify. T8–T10, evidence. Depends: A1–A6