aitools: recover from a removed built-in marketplace on plugin install

[dustinvannoy-db]

Why

When databricks aitools install installs the databricks plugin for Claude Code, the plugin is served from Claude's built-in claude-plugins-official marketplace. #5771 added a proactive plugin marketplace update before install to fix a stale local copy, but a user who has removed that marketplace entirely still hits an opaque failure at install time:

✘ Failed to install plugin "databricks@claude-plugins-official": Plugin "databricks" not found in marketplace "claude-plugins-official". Your local copy may be out of date...

What

• Detect the removed-marketplace case at the install-failure step via marketplaceMissing(err) (Claude reports Plugin "<id>" not found in marketplace "<name>"), gated by builtinMarketplaceRepairable so only that failure triggers recovery — auth and network errors are left untouched, and we never prompt to re-add a marketplace that is present.
• In an interactive session, prompt the user, then re-add + refresh the marketplace and retry the install once. The built-in marketplace is shared infrastructure, so recovery never records ownership and uninstall never de-registers it.
• In a non-interactive session (or when the user declines or the retry fails), return a BlockedError naming the exact repair commands, rendered from a single repairCommands() helper shared with the prompt so the two can't drift.

Notes for reviewers

• marketplaceMissing matches on the agent CLI's stderr string rather than a sentinel. This is a deliberate exception to the "don't branch on error strings" rule: the claude CLI exposes no error code for this, the same file already does this in claudeAlreadyDisabled, and the match is only used to decide whether to attempt a re-add — the retried install is always the real verdict. The matched string was verified against the live Claude CLI.

Testing

• Unit tests cover: recover-after-prompt, user-declines, non-interactive actionable error, retry-still-fails, and a non-missing (auth) failure that must skip recovery.
• go test ./libs/aitools/... ./cmd/aitools/..., go vet, and ./task lint-q all pass.

This pull request and its description were written by Isaac.

[github-actions]

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• PR number: 6066
• Commit SHA: f6d06849cdd711dfa60af53a2c75288c3c7458a6

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[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @simonfaltum -- recent work in libs/aitools/installer/, libs/aitools/agents/

Eligible reviewers: @Divyansh-db, @MarioCadenas, @Shridhad, @atilafassina, @calvarjorge, @chrisst, @ditadi, @fjakobs, @hectorcast-db, @igrekun, @keugenek, @lennartkats-db, @mihaimitrea-db, @parthban-db, @pffigueiredo, @pkosiec, @rauchy, @renaudhartert-db, @tanmay-db, @tejaskochar-db

_{Suggestions based on git history. See OWNERS for ownership rules.}

[dustinvannoy-db]

@renaudhartert-db would you be able to review this when you have a chance? Thanks!