feat(plugins): let a plugin display its own result set in the message list

[aspuru]

Implements api.search.showMessages(ids, label) as described in #954: a plugin that does its own
retrieval can hand the host a set of email ids and a label, and the host renders exactly those
messages in the message list.

What it does

await api.search.showMessages(ids, "5 messages about the tariff decision");

The list shows those messages and the label appears where a search term normally would, so the
user can see why the list looks that way and clear it like any other search.

Clearing rules

Both are covered by tests, and the second one is the non-obvious one:

• Typing a search replaces a plugin's list. A user's own search always wins.
• Changing folder clears a plugin's list but NOT a user's search. A search is deliberately kept
  across folders and scoped by searchMailboxId. A plugin-supplied set has no equivalent scope, so
  carrying it into another folder would silently reinterpret it — it is dropped instead.

Scope and safety

• ui:message-list permission, declared alongside the existing capabilities.
• Read-only and session-scoped. The host fetches with the user's own JMAP client, so a plugin
  cannot surface a message the user could not already open. Strictly weaker than tabs.categorize,
  which writes keywords.
• Capped at 200 ids per call. A result set is a list a human reads, not a bulk export; the cap
  keeps one call from pulling a whole mailbox through Email/get.

Tests

stores/__tests__/email-store-show-messages.test.ts — 10 tests covering the isPluginList
lifecycle and both clearing rules. Passing against this branch:

✓ stores/__tests__/email-store-show-messages.test.ts (10 tests) 7ms
  Test Files  1 passed (1)
       Tests  10 passed (10)

eslint reports no new problems; the warnings it emits on this tree are pre-existing.

Notes

Based on 1.9.2. Happy to change the name, the signature, or the clearing semantics — the shape
here is what fell out of using it rather than a strong opinion, and I'd rather match your
preference than land it as-is.