Add "Inki" Claude Code plugin: research, write, review, submit Strapi Docs update

[pwizla]

This PR introduces the Inki Claude Code plugin at claude-plugins/inki/, exposing 21 skills across four families (Research, Write, Review, Submit) to help maintain the Strapi documentation.

• Migrates agents/prompts/, agents/templates/, agents/authoring/ into claude-plugins/inki/references/ so the plugin is self-contained. agents/ is kept as a pointer README so users who do not install the plugin, but cloned the repo, will still have their AI agents understand where files live.

• Adds a self-hosted marketplace manifest at .claude-plugin/marketplace.json so contributors can install with /plugin marketplace add strapi/documentation then /plugin install inki@strapi-documentation

• Keeps git-rules.md and 12-rules-of-technical-writing.md at the repo root as the canonical versions (still readable for contributors who don't use Claude Code). A GitHub Action auto-syncs them into the plugin's references on every PR and blocks edits made only to the plugin copy, to keep root and plugin in sync.

• Updates AGENTS.md, .cursor/rules/, and .github/prompts/ to reference the new claude-plugins/inki/references/ paths.

This PR opens the plugin as a draft for staged rollout.

[pwizla]

TODO before merging this PR

This PR migrates agents/ content into claude-plugins/inki/references/. As long as this PR is not merged, any change made to agents/** on other branches will be silently lost when this PR finally merges (because agents/ is removed here).

Required sync step before marking ready

1. Run this query to find PRs that touched agents/** between this PR's creation and its merge:

   gh pr list --repo strapi/documentation --state all --limit 200 \
     --json number,title,mergedAt,author,files \
     --jq '[.[] | select(.files[].path | startswith("agents/")) | {number, title, mergedAt, author: .author.login}]'

2. For each PR (open or recently merged) that touched agents/**, manually mirror the diff into claude-plugins/inki/references/:
   • agents/prompts/<file>.md → claude-plugins/inki/references/prompts/<file>.md
   • agents/templates/<file>.md → claude-plugins/inki/references/templates/<file>.md
   • agents/authoring/<file>.md → claude-plugins/inki/references/authoring/<file>.md
3. Commit the mirrored changes to this branch with a message like Mirror PR #<num> changes to inki/references/ before merge.
4. Verify with diff that the inki/references/ versions match what's expected.

Already mirrored on this branch

• PR Update Users & Permissions documentation #3169 (Update Users & Permissions documentation) — mirrored in commit 22b2b1fb (changes to AGENTS.cms.features.md and style-checker.md).

After this PR merges

The agents/ folder is replaced by a single pointer README. From then on, all changes go directly to claude-plugins/inki/references/. This mirror dance disappears.