fix(bin): name local-only delivery base in Definition of done

[rajput-hemant]

Summary

• Thread optional --base <branch> through bin/fm-brief.sh into fm_dod_block / fm_ship_rule_one so a local-only brief names the same delivery target in Setup, Rule 1, and Definition of done.
• Preserve today's default-branch wording when --base is omitted; bin/fm-promote.sh keeps working with no base argument.
• Cover both cases in tests/fm-brief.test.sh. With the old DOD wording restored, a --base feat/vue-3 brief still said merges it into local main while the contract line recorded base=feat/vue-3.

Test plan

• bash tests/fm-brief.test.sh
• bin/fm-lint.sh bin/fm-dod-lib.sh bin/fm-brief.sh
• Fail-proof: temporarily hardcode local-only DOD back to main, scaffold with --base feat/vue-3, confirm DOD still named main

Follow-up (out of scope)

• Spawn should record and refuse a mismatched --base against the brief contract line (see open PR feat(bin): make a task's delivery target branch part of its contract #4436 for the wider delivery-target contract).
• Local-only Setup could prefer the local refs/heads/<base> before fetching when the local base is ahead of origin.
• direct-PR / no-mistakes DOD could add an explicit "PR targets <base>" note when --base is set.

[kunchenguid]

Speaking as Kun's firstmate:

First triage on HEAD 578b885d022ab5fd2a2ef1300d24329f7ff7c8a8. Diff reviewed: threads optional --base through bin/fm-brief.sh into fm_dod_block / fm_ship_rule_one / Setup so local-only briefs name the same delivery target in Rule 1 and Definition of done; omitted --base keeps today's main wording; validation is syntax-only via fm_delivery_base_validate. Overlaps open #4436 (wider delivery-target contract including spawn/teardown; that PR is CONFLICTING) — this is the narrower DOD-wording slice; help this PR, do not open a competing one.

VISION per-rule:

1. One captain, one interface — aligns (honest delivery target in worker brief).
2. Authority explicit — aligns (--base is explicit opt-in; no inferred base).
3. Scripts own mechanics / agents own judgment — aligns (deterministic contract line + label owner).
4. Restart is a non-event — not in play (scaffold text only).
5. Delegation with a spine — aligns (delivery contract names the landing branch before work starts).
6. Fleet outlives any vendor — not in play.
7. Scope — aligns (brief/DOD command-layer contract).

contract-class: opt-in — new --base stays off unless the operator passes it; unconfigured scaffolds unchanged.

CI/NM: first-time fork runs approved this pass (34953405764 CI, 34953405881 Require no-mistakes) — waiting on green. attestation: MISSING (no no-mistakes attestation block / bound head_sha in body). mergeable: MERGEABLE/UNSTABLE. workflow-zero: yes. security: none.

Stamp waiting-author. Please bind a no-mistakes attestation to this HEAD. Merge-eligible only after attestation MATCH + green CI/NM. Not escalating while attestation is missing.