feat: add plugin marketplace published from source via dist branch

[Marinski]

Screenshots

Claude Code

VS Code

Opencode (needs extra plugin to enable support for agents and skills auto discovery from Claude/Codex/VSCode etc. https://github.com/Marinski/opencode-skill-autodiscovery)

• as slash command

• tab-switched

Summary

Adds a Claude-format plugin marketplace: each division ships as an installable
skills plugin (agency-<division>) for Claude Code, Claude Desktop, VS Code
(Agent Plugins, Preview), and GitHub Copilot CLI.

Per-division layout:

• skills/<stem>/SKILL.md — portable Agent Skill (kebab-case name matching the
  dir, required by VS Code; description taken from the agent frontmatter).
• agents/<stem>.md — byte-identical copy of the source agent, usable as a
  Claude Code sub-agent.
• README.md — per-plugin readme for VS Code's detail pane.
• .claude-plugin/plugin.json — identity metadata only (no component-path
  fields; the runtimes discover skills/ and agents/ by default directory).

Source/dist split (matching the repo's "Committed build output" rule in
CONTRIBUTING):

• The generated plugins/ tree and .claude-plugin/marketplace.json are never
  committed to main. They are gitignored and published by CI to the plugins
  dist branch, so consumers point at <owner>/agency-agents#plugins.
• Division folders stay the single source of truth, byte-checked by the
  validator.

Changelist

• scripts/build-marketplace.py — generates the dist from divisions.json and
  the on-disk roster.
• scripts/check-plugin-marketplace.sh — validates a fresh temp build and
  guards that main never tracks generated output.
• scripts/publish-marketplace.sh — builds, validates, and force-publishes the
  dist to the plugins branch.
• .github/workflows/check-plugin-marketplace.yml — PR gate (fresh temp build +
  claude plugin validate + registration smoke test).
• .github/workflows/publish-plugin-marketplace.yml — publish the dist on push
  to main.
• scripts/check-divisions.sh — treats plugins/ as non-division output.
• .gitattributes — keeps generated .claude-plugin JSON LF on all platforms.
• Docs (README Option 5, integrations, CONTRIBUTING) updated for the #plugins
  consumption path.

Verification

• Single commit on top of current upstream main (5457373); no merge commits.
• No agent sources or convert.sh outputs are touched, so
  test-convert-outputs.sh is unaffected (no manifest drift from this PR).
• Fork CI green on the head commit: divisions, tools, runbooks, plugin-marketplace
  consistency (incl. claude plugin validate + registration smoke test),
  test-installer, and the publish job.