fix(desktop): let anyone mention an agent that is already in the channel

[mattbalza]

Problem

An agent in a channel is addressable only by its operator. On every other
device mentionableAgentPubkeys is empty for that agent while the relay
directory (kind:10100) lists it — and shouldHideAgentFromMentions read
exactly that combination (member, directory-present, not locally mentionable)
as an explicit not-invocable signal, and hid it.

The result is a participant nobody in the room can talk to: it posts messages,
it shows in the member sidebar, and typing @ does not offer it.

This is what our @erp and @codex agents look like to every user on the
relay except the one machine that manages them.

Fix

Channel membership already authorises every other participant; make it
sufficient here too.

// Channel membership is sufficient for people and agents alike. Whether an
// agent is managed on this device only governs local lifecycle controls.
if (!isAgent || isMember) return false;
return !mentionableAgentPubkeys.has(normalizePubkey(pubkey));

Whether an agent is managed on this device governs local lifecycle controls
(start, stop, attach) — not whether a peer may speak to it. With the
membership clause isAgentIdentityInManagedList no longer answers "is this
managed here" but "may this be surfaced at all", so it is renamed
isAgentIdentityDiscoverable.

The directoryAgentPubkeys memo in useMentions had no other consumer and is
removed.

Why this is safe

DMs are unaffected. isMember: true is set in exactly one place — the
members loop in useMentions, from the channel's own member list. Every
other candidate source hard-codes isMember: false, and the coalescer only
ORs. An agent that is not party to a conversation is still gated by the
managed-list branch.

Nothing is newly disclosed. Channel members are already listed in the
sidebar; this only makes one of them typeable.

Tests

The existing tests stopped at the predicate, which is how it stayed possible
to relax it without knowing whether a mention actually reaches the agent.
The new agentMentionComposedPath.test.mjs walks the whole path a picked
mention travels — eligibility → mentionCandidateLabel →
mapMentionCandidateToSuggestion → snapshotDraftMentionRefs →
messageMentionPubkeys — and asserts at the only stage the agent can observe,
the emitted p tags:

• a channel-member agent managed on another device reaches the p tag, and is
  not flagged notInChannel
• an agent in nobody's channel and nobody's managed list produces no suggestion
  and no p tag
• a locally managed non-member agent is still suggested and still routes
  (prior behaviour preserved)
• a DM addresses its own participants; an uninvolved agent never appears

Test plan

cd desktop
pnpm test        # 3876 pass, 0 fail
npx tsc --noEmit # clean
npx biome check src/

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