[BUG] Repo-committed extraKnownMarketplaces + enabledPlugins never processed in claude.ai/code cloud sandboxes (hooks from the same settings.json work)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet (closest relatives hit different links of the chain: #16870 is the managed-settings tier being ignored locally, #76759 fails after a registration already exists, #77184 concerns pruning of declared marketplaces)
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code (2.1.211)

What's Wrong?

A private repo commits this .claude/settings.json on its default branch (sanitized; the marketplace source repo is the same private repo the session runs in):

{
  "extraKnownMarketplaces": {
    "myteam": {
      "source": { "source": "github", "repo": "myorg/myrepo" },
      "autoUpdate": true
    }
  },
  "enabledPlugins": {
    "myplugin@myteam": true
  },
  "hooks": {
    "PostToolUse": [
      {
        "hooks": [
          { "type": "command", "command": "python \"$CLAUDE_PROJECT_DIR/tools/hook.py\"" }
        ]
      }
    ]
  }
}

In a claude.ai/code cloud session on that repo (booted via claude --cloud from a local checkout of the default branch; sandbox: Linux, Claude Code 2.1.211):

• The hooks key from this file IS honored: the PostToolUse hook fires repeatedly, $CLAUDE_PROJECT_DIR expands, hook output is injected. So the committed file is read and trusted.
• The extraKnownMarketplaces key is never processed: ~/.claude/plugins/known_marketplaces.json does not exist at any point in the session, before or after any command. ~/.claude/plugins/ contains only an empty installed_plugins.json ({"version": 2, "plugins": {}}) whose mtime predates the session (baked into the sandbox image). No marketplaces/ directory, no cache/ directory.
• Consequently enabledPlugins cannot resolve: none of the plugin's skills appear in the session's skill surface, and /myplugin:... commands are unknown.
• claude plugin marketplace update myteam inside the sandbox fails with Marketplace 'myteam' not found. Available marketplaces: (empty list), confirming zero registered marketplaces.

The plugin docs recommend exactly this mechanism for team distribution: "Declare them in the repo's .claude/settings.json instead" (plugins/marketplace docs). Cloud sandboxes are the environment that guidance targets most (no user-scope state), and they are where the key does nothing.

A possibly related observation from a local machine (Windows 11, 2.1.211, same repo): the declared autoUpdate: true is never merged into an already-registered known_marketplaces.json entry for the same marketplace either (that entry was originally created by a manual claude plugin marketplace add). So across both surfaces I have not observed project-settings extraKnownMarketplaces ever create or modify a marketplace registration; only manual CLI registration works. If a consent gate for project-settings marketplaces cannot prompt in the cloud sandbox and silently declines instead, that would explain the cloud half; in that case the decline should be loud.

What Should Happen?

At session start in a cloud sandbox, marketplaces declared in the repo's committed .claude/settings.json should be registered and (with autoUpdate: true) fetched, and enabledPlugins should install and surface the plugin's skills. If a consent/trust gate intentionally blocks this in headless or cloud contexts, the refusal should be surfaced loudly instead of being silently skipped while sibling keys (hooks) from the same file are honored.

Error Messages/Logs

$ claude plugin marketplace update myteam; echo "exit=$?"
Updating marketplace: myteam...Failed to update marketplace(s): Marketplace 'myteam' not found. Available marketplaces:
exit=1

$ cat ~/.claude/plugins/known_marketplaces.json
cat: /root/.claude/plugins/known_marketplaces.json: No such file or directory

$ cat ~/.claude/plugins/installed_plugins.json
{
  "version": 2,
  "plugins": {}
}

$ ls -la ~/.claude/plugins/marketplaces/
ls: cannot access '/root/.claude/plugins/marketplaces/': No such file or directory

Steps to Reproduce

1. In a private repo, commit the .claude/settings.json above on the default branch (marketplace source = the repo itself; any plugin with at least one skill; optionally a PostToolUse hook as a control).
2. From a local checkout of the default branch, boot a cloud session: claude --cloud "inspect plugin state".
3. In the cloud session: check the skill surface for the plugin's skills; ls -la ~/.claude/plugins/; cat ~/.claude/plugins/known_marketplaces.json; run claude plugin marketplace update <name>.
4. Observe: the control hook fires, but no marketplace registration exists, the update command reports zero known marketplaces, and no plugin skills load.