feat(slack): refuse unresolved own-team members as their own state

[yzxcj797]

Fixes defects 1 and 2 of #626 (the classification collapse and the invisible refusal); the ok:false propagation (1b) and event-driven refresh (3) are separate follow-ups per the issue's fix ladder.

Defect 1 — the third state

classifyUser folded three states into two: confirmed-external → guest; confirmed-internal-resolved → allowed; and own-team-but-email-invisible → also "guest". An org owner with a just-set email masqueraded as an external guest for the directory-propagation window, with every log line and the user-facing copy asserting "external".

Now ActorAssertion carries principalUnresolved: true for that third case:

• Native flags still decide isExternalGuest — a restricted/other-workspace/stranger member with a visible email remains an email-keyed external guest exactly as before (pinned by the pre-existing tests, now joined by a "native evidence wins" case). The third state never masks the flags.
• Email keying is unchanged for everyone with a visible email.
• slack-id mode never sets it (no email resolution exists there).

Defect 2 — the refusal is its own state, and observable

The turn-handler gate refuses principalUnresolved ahead of the external-guest gate — equally fail-closed (never a fallback Slack-ID principal, which would mint a second internal identity with wrong scope/grants, exactly what email mode exists to prevent), but:

• the sender sees "I couldn't verify your team membership (your email isn't visible to me). This usually resolves within a few minutes of the directory refreshing; try again shortly" — naming the actual condition and its actual remedy, instead of "this conversation isn't fully internal", which sent the owner looking for a channel-setting problem;
• every refusal logs on the success path — the new principalUnresolved refusal warns with user/channel/timestamp, and the external-guest refusal (which previously left no trace unless postEphemeral threw) now warns too. In the incident, H1 (path never ran) / H2 (ephemeral posted but not displayed) / H3 (hung await before the ephemeral) were indistinguishable precisely because a successful refusal was invisible; the success-side log line makes the next one decidable.

Tests

• Updated: "email mode fails closed to guest when a member has no visible email" → asserts isExternalGuest: false + principalUnresolved: true. Fails on main (guest).
• New: "native external evidence still wins over unresolved" — restricted + email-less reads as external guest, principalUnresolved unset. Pins that the third state cannot mask the flags.
• New: "slack-id mode never marks principalUnresolved".

slack-identity 27/27; neighbors external-slack-participants / slack-index.integration / identity-offboarding 53/53 (all three suites identical on clean main); tsc --noEmit clean. The refusal-gate copy and logging are exercised through the integration suite's mention fixtures.

Follow-ups (not included, per the issue's ladder)

• ok:false from users.info preserved at classifyActor and the direct-intake path, distinguishing "lookup failed" from "resolved but email-less" — the transient-failure half of defect 1.
• user_change/team_join manifest subscriptions + cache eviction — defect 3; needs the app-reinstall note the issue mentions.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}