fix(desktop): scope the mention picker to the current relay's managed agents - #5720

managed-agents.json is one file per install and every record keeps the relay URL its keypair was minted against. Changing a community's relay URL mints new keypairs and appends them without removing the old rows, and useMentions read the whole file — so the @ picker showed one entry per historical relay: three communities'-worth of Fizz/Honey/Bumble under different npubs, all labelled "managed by you · not in channel". None of the stale ones are reachable on the relay the user is connected to.

Filter the list to the active community's relay once, and derive the name, persona-id, and pubkey lookups (and the mentionableAgentPubkeys set built from them) off the scoped list.

managedAgentsForRelay compares canonically via the existing canonicalRelayUrl, so localhost vs 127.0.0.1, a default port, and a trailing slash still match the same relay. It deliberately fails open in two places, each covered by a test:

• an unknown or unparsable active relay URL returns the list untouched, so a surface that cannot say where it is shows every agent rather than none;
• a record whose own relayUrl will not parse is kept, so a real agent is never hidden on the strength of a URL we failed to read.

This is the issue's second suggested fix (scope the picker). It does not clean up managed-agents.json — the stale records stay on disk and stay startable from the Agents view, which felt like the wrong thing to decide inside a mention picker. Happy to follow up with the removal/archive path if you would rather fix it at the source.

Verified locally in desktop/: pnpm typecheck, pnpm check, and pnpm test (4721 pass, including the 5 new ones). I did not build or run the Tauri app.

Closes #5488