fix(desktop): keep live foreign agents in mention and add-member autocomplete

[neptun787]

Problem

Since #2149, the composer's @-mention autocomplete and the members-sidebar add-member search drop every concrete agent identity that is not in the current user's local managed Agents list — including live, invocable agents owned by another desktop that are members of the current channel.

useMentions.addCandidate returns early via isAgentIdentityInManagedList(...) before shouldHideAgentFromMentions(...) (#1611) ever runs, so the whole "member / invocable / directory-exclusion" decision table from #1611 is unreachable for foreign agents. The same gate sits in MembersSidebar.tsx.

Concrete repro (two desktops, one relay):

1. Desktop B manages agent GADS MacMini and adds it to a shared channel.
2. On desktop A (different owner), open that channel and type @gads.
3. No suggestion appears — in the channel or in a 1:1 DM with the agent — even though the agent is a channel member, online, and listed in the relay agent directory (kind:10100). Send-time mention extraction (extractMentionPubkeys) scans the same candidate list, so typing the full name doesn't p-tag it either: the agent is effectively unmentionable from any other device.

Meanwhile #2149's goal is real: stale p-tag/member identities (superseded agent keys) should not resurface.

Fix

Rename the gate to isKnownLiveAgentIdentity and let a relay agent directory entry (kind:10100) count as live evidence alongside the local managed list. Everything else is unchanged — surviving foreign agents still flow through shouldHideAgentFromMentions, so explicitly non-invocable ones stay hidden.

candidate	before	after
person	show	show
locally managed agent	show	show
foreign agent, in directory, invocable (shared channel / allowlist)	dropped	show
foreign agent, in directory, not invocable ("Fizz", #1611)	dropped	hidden by shouldHideAgentFromMentions (as designed)
stale identity — no directory entry, not managed (#2149's Bumble)	dropped	dropped

Applied to both call sites (useMentions.ts, MembersSidebar.tsx); MembersSidebar already had relayAgentsQuery in scope and in the memo deps.

Tests

• agentAutocompleteEligibility.test.mjs: reworked the gate's test to cover person / managed / directory-listed-foreign / stale branches
• pnpm test → 3906/3906 pass
• tsc --noEmit → 0 errors
• biome check on the 4 changed files → clean

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