chore: convert forum announcement instruction as a proper agent skill

.gitattributes

@@ -8,7 +8,6 @@
 .gitignore     export-ignore
 
 # admin files
-.ai/               export-ignore
 .github/           export-ignore
 admin/             export-ignore
 contributing/      export-ignore

.github/skills/README.md

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+# Skills
+
+This directory contains workspace agent skills for maintainers.
+
+## Available Skills
+- `forum-announcement`: Create CodeIgniter4 release forum announcements in myBB format.
+
+## Structure
+- `.github/skills/<skill-name>/SKILL.md`: Skill metadata and workflow.
+- `.github/skills/<skill-name>/references/`: Optional supporting docs loaded on demand.
+- `.github/skills/<skill-name>/assets/`: Optional templates and reusable files.
+- `.github/skills/<skill-name>/scripts/`: Optional executables for automation.
+
+## Usage
+1. Open Copilot Chat in this workspace.
+2. Invoke `/<skill-name>`.
+3. Provide inputs, for example:
+```
+/forum-announcement 4.7.0
+```
+
+## Maintainer Notes
+- Keep `name` in `SKILL.md` identical to its folder name.
+- Keep the `description` keyword-rich so the skill is discoverable.
+- Use references only when needed; avoid duplicating guidance between `SKILL.md` and `references/`.

.github/skills/forum-announcement/SKILL.md

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+---
+name: forum-announcement
+description: 'Create CodeIgniter4 release forum announcements in myBB format. Use when preparing release posts, dual-version announcements (maintenance + major), changelog summaries, contributor acknowledgements, upgrade guidance, and final forum-ready text files.'
+argument-hint: 'Target release version(s), e.g. "4.6.5 and 4.7.0" or "4.7.1"'
+user-invocable: true
+---
+
+# Forum Announcement
+
+Create a forum-ready CodeIgniter4 release announcement using myBB markup.
+
+## When to Use
+- Publish a new CodeIgniter4 release announcement on the forum.
+- Convert changelog and RST details into a structured, readable post.
+- Produce a dual-release announcement with maintenance-first ordering.
+
+## Inputs
+- One version (`4.x.y`) or two versions (`maintenance`, then `major`).
+
+## Output Naming
+- `v{VERSION}` is a placeholder and must be replaced by an actual version string.
+- Single release example: `v4.7.1-announcement.txt`.
+- Dual release example (`4.6.5` + `4.7.0`): use the major version filename `v4.7.0-announcement.txt`.
+- Save the file at repository root.
+
+## Procedure
+1. Read `CHANGELOG.md` for release summary and PR references.
+2. Read `user_guide_src/source/changelogs/v{VERSION}.rst` for each target version.
+3. Determine release mode:
+    - Single release: one `4.x.y` version.
+    - Dual release: maintenance + major; maintenance is presented first in the post.
+4. Draft the post with this structure (remove sections that do not apply):
+    - Title.
+    - Intro paragraphs.
+    - Release links.
+    - Which version to use (dual release only).
+    - Maintenance release section (dual release only).
+    - Highlights and new features.
+    - Notable enhancements.
+    - Security and quality.
+    - Breaking changes.
+    - Other notable changes.
+    - Contributor thanks.
+    - Upgrade links, issue reporting link, closing.
+    - AI-assistance disclosure.
+5. Apply myBB formatting and escaping rules in this file.
+6. Save final content as `v{VERSION}-announcement.txt` at repository root, replacing `{VERSION}` with the actual version.
+
+## Required Announcement Template
+```text
+[size=x-large][b]CodeIgniter {VERSION} Released![/b][/size]
+
+Introduction paragraph(s)
+
+[url=RELEASE_URL]GitHub Release[/url]
+[url=CHANGELOG_URL]Changelog[/url]
+
+[hr]
+
+[size=large][b]Highlights & New Features[/b][/size]
+
+[hr]
+
+[size=large][b]Notable Enhancements[/b][/size]
+
+[hr]
+
+[size=large][b]Breaking Changes[/b][/size]
+
+[hr]
+
+[size=large][b]Thanks to Our Contributors[/b][/size]
+
+[hr]
+
+[url=UPGRADE_GUIDE_URL]Upgrade Guide[/url]
+[url=ISSUES_URL]Report Issues[/url]
+
+[hr]
+
+[size=small][i]Note: This announcement was created with the assistance of GitHub Copilot (GPT-5.3-Codex).[/i][/size]
+```
+
+For dual releases, adapt the title and body to include both versions, with maintenance version first, plus a dedicated "Which Version Should I Use?" section.
+
+## myBB Rules
+- Use `[b]`, `[i]`, `[size=x-large]`, `[size=large]`, `[size=small]`, `[url=...]`, `[list]`, `[list=1]`, `[*]`, and `[hr]`.
+- Use double backticks for inline code-like text when needed.
+- Escape colon patterns that may trigger myBB emoticons by replacing `:` with `:` in code-like snippets.
+
+## Content Quality Rules
+- Tone is professional, friendly, and community-facing.
+- Do not use emojis.
+- Verify details against RST changelog entries before finalizing.
+- Explain breaking changes with old behavior versus new behavior.
+- Include a "Removed Deprecated Items" subsection when applicable.
+- For maintenance bug-fix summaries, prefer ordered lists with concise before/after statements.
+- Thank contributors by GitHub username.
+- Include upgrade links and issue reporting links.
+
+## Version Guidance Rules
+- Dual releases must list maintenance before major.
+- The "Which Version Should I Use?" section is required for dual releases.
+- In version guidance, present the lower-PHP-support option first.
+- For single releases, omit the "Which Version Should I Use?" section.
+
+## Self-Check
+- Maintenance version is presented before major version.
+- PHP-version guidance and upgrade links are included.
+- Breaking changes are clearly explained.
+- Final AI-assistance disclosure is present and should be adjusted to the actual AI model used if different from the template.
+- Output filename is concrete and does not include braces.