Publish lease-backed work claims so preflight sees active drafts

[catoncat]

Problem

Mainline intentionally keeps drafting intents local-only. That protects unfinished turns and avoids publishing unstable fingerprints, but it leaves a coordination blind spot:

• mainline status can discover sibling worktree drafts on the same machine.
• mainline preflight only compares the active work against the shared proposed index.
• mainline sync cannot make drafting work visible to another clone or machine.

As a result, several agents can start overlapping work and all pass preflight until one of them seals and publishes. Sealing early is not a good workaround: it freezes an unstable intent and contributes to the same-branch staged-proposal problem described in #186.

Desired model

Add a lightweight, lease-backed work claim that is shared separately from the local draft:

claim (shared, renewable, minimal)
  → drafting (local implementation detail)
  → finalize/seal (one immutable intent)
  → proposed
  → Draft PR

A work claim is coordination state, not a new immutable intent status.

Proposed interface

Keep the external interface small:

• mainline claim — announce or refresh the active branch intent.
• mainline unclaim — explicitly release it.
• mainline preflight / sync — consume active local and remote claims automatically.

The implementation may use append-only claim/renew/release events on shared Mainline refs.

A claim should contain only:

• intent id
• repository identity
• actor plus session/worktree identity
• branch/thread
• base commit
• concise goal
• partial fingerprint (goal keywords and currently known files)
• created/updated/expires timestamps

Do not publish draft turns, seal summaries, or other unfinished detail.

Important invariants

• Claims expire unless renewed, so crashed sessions do not create permanent blockers.
• Exact active intent id is excluded from self-overlap.
• Different sessions sharing the same human actor id must still see each other; conflict detection must not skip all same-actor work.
• Local sibling worktree drafts should be surfaced immediately even before remote claim transport succeeds.
• A failed claim publish is visible as a warning/pending state; local work can remain possible offline.
• Seal/propose supersedes or releases the claim automatically.
• Abandon releases the claim.
• Claims warn by default; they block only when file/scope evidence is strong enough under an explicit policy.

Preflight behavior

Add a distinct overlap kind such as active_claim_overlap containing:

• intent id
• branch/thread
• actor/session
• matched files or goal keywords
• lease freshness
• local sibling vs remote source

Recommended next actions should be inspect/coordinate/re-scope, not seal the other agent's draft.

Relation to #186

This issue owns early work visibility. #186 owns same-branch proposal lineage and final PR closeout.

Together they avoid both failure modes:

1. hidden drafts that duplicate work;
2. premature repeated seals that leave noisy staged proposals.

Acceptance criteria

• Starting a non-trivial intent can produce a shared claim without sealing it.
• Another worktree on the same machine sees the claim in its next preflight.
• Another clone sees the claim after sync.
• Same-actor/different-session claims are not silently excluded.
• Expired/released claims stop appearing.
• Sealing/publishing leaves one immutable intent and removes the active claim.
• No draft turns or unfinished seal payload are placed in shared state.
• Tests cover local sibling, remote clone, lease expiry, release, same-actor concurrency, publish failure, and claim-to-proposal transition.

[catoncat]

已提交 Draft PR #200，作为本 issue 的第一阶段：让同一 clone 中其他 worktree 的 drafting intent 出现在 preflight --json，并对同文件或同目标的重叠给出 advisory warning。

该 PR 刻意不发布跨机器 claim，也不改变 draft local-only 的语义；remote lease-backed claim 仍由本 issue 继续承载。

已做 focused tests、CLI preflight tests、相关 package go vet 和本地 wrapper live readback；未运行 full/fuzz/property suite。