Test the integrations catalog and picker

[nrodd]

Chunk 4 of the test plan. src/integrations.test.ts, 75 tests. No source changes.

Based on #42 (the harness). GitHub will retarget this to main when that merges. Independent of #43, #44 and #45 — different files.

Where the weight went

The picker only steers the agent — anything it misses, the agent's own Step 2 explore still detects. So this doesn't chase prompt coverage. It leans on two things that can actually break a run:

Catalog consistency. The catalog is hand-maintained and feeds three places at once: the picker, package detection, and the linkage examples pasted into the agent prompt. These run per-entry so a failure names the tool:

• unique ids and labels
• ids lowercase and spaceless — selectIntegrations lowercases before matching, so a capitalised id in the catalog would be silently unreachable from --integrations
• every tool detectable by a package or a window global (Appcues has no npm package)
• every linkageExample non-empty and actually naming the session URL
• no npm package claimed by two tools

--integrations parsing, which is the path CI and scripted runs take: case-insensitive matching, trimming, unknown names routed to other with their original case preserved, empty segments from a doubled comma dropped, and user order preserved rather than catalog order.

The rest

• detectInstalledIntegrations against real temp directories — deps and devDeps, package aliases, several tools at once in catalog order, and the four ways it must return [] rather than throw (no file, unparseable file, missing directory, no deps).
• The collapse. A detection hit confirms the detected set behind a single "add any others?" rather than showing all fifteen options next to one tool. Saying no returns just the detected set; saying yes opens the picker with those pre-checked and hinted detected — but still uncheckable, since a pre-check is a default and not a lock.
• Cancellation at all three prompts raises CancelledError. This is the one place in the flow where a cancel still aborts the run, which is worth having pinned before chunk 7, where the same gesture must not abort.

One recorded behavior

A repeated id in --integrations isn't deduped, so --integrations posthog,posthog lists PostHog twice in the agent prompt. Harmless, and deduping is a behavior change rather than a test, so there's just a test saying so.

Verification

75 tests passing (128 on this branch: chunk 0 plus this one), typecheck clean. Mutation-checked two assertions — dropping the .toLowerCase() in flag matching, and returning an empty set instead of the detected one — and confirmed exactly those tests fail, then reverted.

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Note

Low Risk
Test-only PR with no runtime or CLI behavior changes.

Overview
Adds src/integrations.test.ts (~75 Vitest cases) with no production code changes. Tests pin behavior the CLI and agent rely on: hand-maintained INTEGRATIONS invariants (unique ids/labels, lowercase ids, detectability, linkage examples, no duplicate npm packages), detectInstalledIntegrations against temp package.json trees (including safe empty results on bad/missing input), and selectIntegrations for both --integrations (case/trim, order, other, no prompts) and interactive flows (full picker vs collapsed “any others?” when deps are detected, __other__ free text, catalog ordering, CancelledError on cancel).

Documents current behavior that repeated flag ids are not deduped.

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