Board: connect provider-neutral remote lifecycle observations

[jeffhuber]

Part of #945 and roadmap #900.

Problem

Board also needs trustworthy hosted progress, questions, completion, and failure observations without importing Devin-specific state into its read model or causing provider mutations during refresh.

Scope

Adapt the provider-neutral remote lifecycle from #906 and its safe public_projection into the accepted Board observation contract. Connect existing remote-session, hosted work-order, review, and controller evidence paths through one provider-neutral adapter. Preserve product-specific reason mappings behind the provider adapter; do not add another remote state machine or expose raw provider records.

No Slack-specific mapping or cloud schema changes belong here; those remain #921. Board reads must never dispatch, retry paid create, send a message/answer, approve, terminate/archive, or collect private result prose.

Dependencies

• Board: connect exact local sessions, work, runs, and worktrees #949 — exact local session/work/run/worktree connection, so shared producer/correlation code has one writer.
• Sessions: provider-neutral remote lifecycle and recovery #906 / PR Sessions: add provider-neutral remote lifecycle and recovery #928 lifecycle and recovery contract.

Acceptance criteria

• Provider-neutral fixtures cover pending, active, waiting-for-user, waiting-for-approval, succeeded, failed, cancelled, suspended, creation-unknown, stale, and unavailable observations.
• Assigned, dispatched, provider-observed running, provider-reported stage, completed implementation, verified PR/review evidence, and merged remain separate facts.
• Current PR/head and review freshness are verified independently; a done label cannot become a current-head PASS.
• Provider GET failure preserves the dated last observation and safe reason instead of reporting idle or stopped.
• Refresh is side-effect free for provider and GitHub mutations and does not consume private structured results.
• Hosted Devin passes the adapter contract, while fake-provider tests prove the Board model is not Devin-specific.
• Raw messages, questions/answers, prompts, structured result bodies, provider references, credentials, cost detail without coverage, and private state paths never enter Board output.
• Crash/restart and stale-result/fix-round regressions preserve exact round/head binding.

Initial ownership

Provider-neutral session/events track, sequential after B3. Planned builder: Code Mower Codex.

Code Mower delivery

Produce one independently reviewable PR for this issue. Record the named Code Mower builder, keep one writer on the branch, obtain an independent audit against the exact current head, resolve every P0/P1/P2 finding, and pass focused tests, relevant full tests, privacy checks, normal CI, and code-mower/gate. Upload only allowlisted metadata. No umbrella PR.