chore(devcontainer): migrate AI codespace overlays to Agency plugins

[tylerbutler]

Description

Replace the repoverlay-based overlay system for the AI codespace with
Agency plugins and profiles. Skills, agents, and commands that were
previously injected into the repo at session start via repoverlay now
live permanently in .agency/plugins/ and are loaded on demand via
agency --profile.

• Add agency.toml with ff-oce and nori profiles
• Add .agency/plugins/ff-oce — FF OCE agent and skills; MCPs wired
  via profile rather than agent frontmatter
• Add .agency/plugins/nori — 17 skills, 8 subagents, 1 command
• Strip the nori repoverlay down to CLAUDE.md + repoverlay.ccl only
• Remove the ff-oce repoverlay entirely
• Update agent-aliases.sh: dev() adds --profile nori; oce()
  drops repoverlay and uses --profile ff-oce --agent ff-oce:ff-oce

Key improvements over the previous overlay approach:

• No working directory mutation — files are no longer copied into the repo at session start
• No repoverlay remove step needed when switching contexts
• ff-oce workflow requires no repoverlay at all

Reviewer Guidance

The review process is outlined on this wiki page.

Infrastructure-only change — no Fluid Framework package code is affected.

[github-actions]

Hi! Thank you for opening this PR. Want me to review it?

Based on the diff (269 lines, 77 files), I've queued these reviewers:

• Correctness — logic errors, race conditions, lifecycle issues
• Security — vulnerabilities, secret exposure, injection
• API Compatibility — breaking changes, release tags, type design
• Performance — algorithmic regressions, memory leaks
• Testing — coverage gaps, hollow tests

Toggle the reviewer checkboxes above to adjust, then tick the box below to start:

• Start review

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> fluid-framework-docs-site@0.0.0 ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> fluid-framework-docs-site@0.0.0 serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> fluid-framework-docs-site@0.0.0 check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  287071 links
    1898 destination URLs
    2148 URLs ignored
       0 warnings
       0 errors