docs: remove Proposed and Undecided as valid ADR statuses

[ralphbean]

Summary

• Removes Proposed and Undecided from the set of valid ADR statuses — only Accepted, Deprecated, and Superseded remain
• Moves 11 ADRs from Proposed → Accepted (0028 was handled separately in Scribe agent: replace cron trigger with event-driven trigger when Gemini notes are available #1674)
• Updates lint rules, template, ADR 0001, README, writing-adrs skill, and agent-execution-environment plan

Test plan

• python3 hack/lint-adr-frontmatter passes all 41 ADRs
• make lint passes
• Rebased cleanly on latest main (including the 0028 change)

🤖 Generated with Claude Code

[github-actions]

Site preview

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Commit: 2c891936cabbf91203cd40ebf6c106a2ba0e94fe

[fullsend-ai-review]

Review

Findings

High

• [protected-path] skills/writing-adrs/SKILL.md — This PR modifies a file under skills/, which is a protected path requiring human approval. The PR has no linked issue providing authorization for changes to governance/infrastructure files. The change itself is consistent with the rest of the PR (removing references to Proposed/Undecided statuses), but human review and approval is required for protected-path modifications.
  Remediation: Link an issue authorizing the skills/ change, or obtain explicit human approval on this PR.

Info

• [correctness] All 12 ADRs with status: Proposed on main are updated to Accepted in both frontmatter and ## Status section. The template (0000), lint scripts (hack/lint-adr-frontmatter, hack/util/get-adr-status.sh), README, ADR 0001, writing-adrs skill, and agent-execution-environment plan are all updated consistently. No remaining references to Proposed or Undecided as ADR statuses were found outside the files modified by this PR (other uses of "Proposed" in the repo are regular English, not ADR status values).

[fullsend-ai-review]

See the review comment for full details.