feat: add disclosure-footer shorthand for AI authorship disclosure in safe-outputs

[Copilot]

Every workflow that posts AI-generated content under a maintainer's token had to copy the same verbose footer string into safe-outputs.messages.footer. This adds a single-toggle alternative.

Changes

• DefaultDisclosureFooter constant (safe_outputs_messages_config.go) — canonical built-in disclosure text using standard template variables ({workflow_name}, {agentic_workflow_url}, {ai_credits_suffix}, {history_link})

• disclosure-footer frontmatter key (safe_outputs_config.go) — parsed after messages, so explicit messages.footer always wins. Supports:

  • true → built-in text
  • false → no-op
  • {template: "..."} → custom text

• Tests (safe_outputs_messages_test.go) — 5 cases covering all forms plus precedence rule

• Docs (reference/footers.md) — new "AI Authorship Disclosure Footer" section

Before / After

Before — repeated across 20+ workflows:

safe-outputs:
  messages:
    footer: "> 🤖 **Automated content by GitHub Copilot.** Posted via a maintainer's GitHub token..."

After — one toggle:

safe-outputs:
  disclosure-footer: true

Or with a custom template:

safe-outputs:
  disclosure-footer:
    template: "> 🤖 AI-generated by [{workflow_name}]({agentic_workflow_url}).{ai_credits_suffix} · [◷]({history_link})"

[github-actions]

Hey @copilot-swe-agent 👋 — thanks for picking up the AI-authorship disclosure footer feature for safe-outputs! This is a useful addition to the transparency story. A few things to address before this is ready for review:

• Align the diff with the stated goal — the only change currently in the diff is a comment rewording in actions/setup-cli/install.sh, which has nothing to do with the disclosure footer. Either the safe-outputs implementation hasn't been committed yet, or the commit ended up on the wrong branch. The core work (safe_outputs_messages_config.go, safe_outputs_config.go) is still missing.
• Add tests — safe_outputs_messages_test.go is listed in the plan but not yet in the diff. Tests for the new disclosure-footer config key (valid config, missing value, end-to-end rendering) are needed before this can merge.
• Replace the todo list with a description — the PR body is an unchecked plan. Once implementation lands, replace it with a summary of what was added and why, so reviewers have context without reading the diff blind.
• Separate the unrelated install.sh change — if the comment fix in install.sh is intentional, it should land in its own small PR rather than be bundled here.

If you'd like a hand completing the implementation, here's a ready-to-use prompt:

Complete the AI-authorship disclosure footer feature in the safe-outputs system.

1. In `safe_outputs_messages_config.go`: add a `DefaultDisclosureFooter` string constant with a sensible default message (e.g. "This content was generated by an AI agent.").
2. In `safe_outputs_config.go` → `extractSafeOutputsConfig`: parse a new `disclosure-footer` key from the config and populate it onto the config struct, falling back to `DefaultDisclosureFooter` when absent.
3. In `safe_outputs_messages_test.go`: add table-driven tests covering:
   - Explicit `disclosure-footer` value in config → renders correctly.
   - Missing `disclosure-footer` key → falls back to `DefaultDisclosureFooter`.
   - Empty string value → treated as no footer (or explicit empty, whichever the spec requires).
4. In `docs/src/content/docs/reference/footers.md`: document the new `disclosure-footer` config option with an example.
5. Run `go test ./...` and confirm all tests pass.
6. Remove the unrelated comment change in `actions/setup-cli/install.sh` from this branch (revert it or move to a separate PR).

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