fix: update assignee query to handle empty names and improve description

[pikann]

Problem

The "Open tasks by assignee" dashboard preset showed raw project_members UUIDs as chart labels instead of member names.

Fixes Paca-AI/paca#471

Root cause

The preset's SQL selected pm.id::text as the display label instead of resolving it to a name:

SELECT COALESCE(pm.id::text, 'Unassigned') AS assignee, ...

Fix

Resolve a real display name through both project_members paths:

• Human members: users.full_name, falling back to users.username when full_name is blank (it's NOT NULL DEFAULT '', not nullable, so a plain COALESCE wouldn't have caught this — used NULLIF(full_name, '')).
• Agent members: agents.name.
• No assignee: still grouped under "Unassigned".

SELECT COALESCE(NULLIF(u.full_name, ''), u.username, a.name, 'Unassigned') AS assignee, ...
LEFT JOIN users u ON u.id = pm.user_id
LEFT JOIN agents a ON a.id = pm.agent_id
...
GROUP BY pm.id, u.full_name, u.username, a.name

Also dropped "(member id shown; ...)" from the preset description since it no longer applies.

Testing

Verified against the real schema rather than just by inspection: applied all services/api core migrations plus this plugin's own migrations to an isolated throwaway Postgres container, seeded fixture rows for a human member with a full name, a human member with a blank full name, an agent member, and an unassigned task, then ran the old vs. new query.

• Old query: returned raw project_members UUIDs (reproduces the bug).
• New query: returned Jane Doe, noname (username fallback), Review Bot (agent name), Unassigned — done tasks correctly excluded from counts.

tsc --noEmit passes; no other file in the repo references this preset's query text.

Note: this fixes the preset template for panels created going forward. It does not retroactively update dashboard panels that already saved the old query text (e.g. the reporter's existing panel) — those need the preset re-applied or the panel query edited by hand.