chore: enable fa-platform-plugin (+ drop archived-repo references)

[toini]

What

Adds .claude/settings.json enabling the fa-platform-plugin@tibber-claude-plugins
plugin and registering the tibber/claude-plugins marketplace it is served from.

No .github-private or claude-shared references exist in this repo, so this PR is the settings file only.

Why

PLA-1193
(retire .github-private and claude-shared) — both repos are being retired. The
org-wide engineering conventions they carried now ship as skills inside
fa-platform-plugin (in tibber/claude-plugins), injected by a SessionStart hook
instead of fetched per repo. Neither repo can be archived until nothing references it,
so this sweep lands the plugin reference and removes the stale pointers.

Scope of effect

Per-repo .claude/settings.json only affects Claude Code CLI sessions opened in this
repo. Cowork workspaces already receive the plugin via marketplace sync from
claude-plugins main, so nothing changes there.

This file is exactly what /plugin marketplace add tibber/claude-plugins +
/plugin install fa-platform-plugin@tibber-claude-plugins writes — the interactive
commands are not scriptable across many repos, and committing the file is the
reviewable equivalent.

Part of a mechanical sweep across the fa-platform-owned repo set; one PR per repo.

[tibber-agents]

🤖 tibber-agents[bot] is reviewing this PR. View progress live.

[tibber-agents]

Automated PR review by tibber-agents[bot] · anthropic-claude-opus-4.8. Advisory only — humans decide whether to merge.

Summary

Trivial, config-only change: adds a single valid .claude/settings.json enabling fa-platform-plugin@tibber-claude-plugins and registering the tibber/claude-plugins marketplace. The diff matches the PR description, the file is exactly what /plugin install writes, and I confirmed the repo has no remaining .github-private/claude-shared references (so "settings file only" is accurate). Scope is limited to Claude Code CLI sessions in this repo — no build/runtime/CI impact.

Issues

• ❔ low (.claude/settings.json:9) — I could not independently verify that the tibber/claude-plugins marketplace repo / fa-platform-plugin plugin resolves (the API returned 404), but this is almost certainly because the repo is outside this review's token scope rather than a real gap — the PR body describes it as an actively-synced marketplace.
  Suggested: No action needed if the marketplace repo exists and is reachable to Claude Code CLI users; flagged only so a human with access can sanity-check the repo reference once.

Conventions consulted

• tibber/.github-private:org-wide-instructions.md
• README.md
• .github/CODEOWNERS

[toini]

Thanks — that one is a false positive from the review's token scope, no action needed.

tibber/claude-plugins exists, is not archived, and plugins/hope/fa-platform-plugin/ is present on main; the marketplace is actively synced into Cowork workspaces from that branch. The 404 is the repo being outside the review App's scope, not a dangling reference.