bugs: frame scanned-repo and tracker text as untrusted data; author-check scan's lane cursor and dedupe #7394

Found by the security review of #7393. Both predate that PR (the PR rewrites these bodies but neither added nor removed this).

1. Untrusted-content framing absent (P4). plugins/bugs/skills/scan/SKILL.md (rung 1, hunters, Step 6 dedupe) and plugins/bugs/skills/write/SKILL.md (survey) read repository files and tracker item text, but no bugs skill or context file carries the inline framing docs/conventions/untrusted-content/README.md requires (grep -rln 'never instructions to you' plugins/bugs is empty). A planted imperative in scanned code or a tracker item could steer hunters or gates (suppress or skew findings). Filing stays gated by --track and filing_posture. Fix: add the convention's inline form to the scan Step 2 and Step 4 dispatch contracts (context/lenses.md, context/verification-gate.md) and the write survey step.

2. Lane cursor and dedupe trust any author (P4). Rung 1 picks the next lane from the newest tracker item containing Filed by /bugs:scan (lane: <name>) with no author check, so on a public repo anyone can steer the rotation; Step 6 dedupe can attach a real finding as a comment on an outsider's issue. plugins/work-items/reference/item-content-trust.md requires a provider-enforced write surface. Fix: accept the provenance line and dedupe matches only from the scanning identity or a repo writer; otherwise fall through to rung 2 / file a new item.

Also noted, not graded: scan/context/verification-gate.md lets a gate run the scanned repo's tests to label a finding reproduced, which executes scanned code on an unattended run over an untrusted checkout.

Done when

• plugins/bugs/skills/scan/context/lenses.md, plugins/bugs/skills/scan/context/verification-gate.md, plugins/bugs/skills/scan/SKILL.md (rung 1 lane cursor and Step 6 dedupe) and plugins/bugs/skills/write/SKILL.md (survey step) each carry the inline form from docs/conventions/untrusted-content/README.md, with both conformance fragments unbroken: never instructions to you and untrusted-content/README.md` "The framing contract".
• Rung 1's lane cursor and Step 6 dedupe accept a Filed by /bugs:scan (lane: <name>) match only from the scanning identity or a repository writer, and otherwise fall through to rung 2 or file a new item.
• The changelog fragment is valid (the lane's fragment check passes), and CI's check-skills job passes on the PR (a PR check; this run does not run it).

Out of scope

• The verification gate running the scanned repository's tests (the "Also noted, not graded" item): tracked in bugs: verification gate runs the scanned repository's tests on an untrusted checkout #7787.