feat(agent): replace hard delete with archive/restore (#346)

* feat(agent): replace hard delete with archive/restore

Replace agent deletion with soft archive pattern. Archived agents
are preserved in the database with all historical references intact
but cannot be assigned, mentioned, or trigger tasks.

Backend:
- Add archived_at/archived_by columns to agent table (migration 031)
- Replace DELETE /api/agents/{id} with POST /api/agents/{id}/archive
- Add POST /api/agents/{id}/restore endpoint
- ListAgents excludes archived by default (?include_archived=true to include)
- Skip archived agents in task triggers (on_assign, on_comment, on_mention)
- Block assignment to archived agents
- Cancel pending tasks on archive
- New events: agent:archived, agent:restored (replacing agent:deleted)

Frontend:
- Agent type includes archived_at/archived_by fields
- Mention autocomplete and assignee picker filter out archived agents
- Agent list shows archived agents with muted styling
- Agent detail shows archive banner with restore button
- Delete button replaced with Archive button and updated confirmation dialog
- API client: archiveAgent/restoreAgent replace deleteAgent

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(agent): self-review fixes for archive feature

- Fix: workspace store now fetches agents with include_archived=true
  so archived agents are actually visible in the frontend (the archived
  UI was dead code before — ListAgents excludes archived by default)
- Fix: add error logging for CancelAgentTasksByAgent in ArchiveAgent
- Fix: add idempotency guards — return 409 Conflict when archiving
  an already-archived agent or restoring a non-archived agent
- Fix: revert unnecessary extra GetAgent query in ReconcileAgentStatus
  (archived agents won't have running tasks after CancelAgentTasksByAgent)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/db/queries/agent.sql

@@ -1,5 +1,10 @@
 -- name: ListAgents :many
 SELECT * FROM agent
+WHERE workspace_id = $1 AND archived_at IS NULL
+ORDER BY created_at ASC;
+
+-- name: ListAllAgents :many
+SELECT * FROM agent
 WHERE workspace_id = $1
 ORDER BY created_at ASC;
 
@@ -37,8 +42,15 @@ UPDATE agent SET
 WHERE id = $1
 RETURNING *;
 
--- name: DeleteAgent :exec
-DELETE FROM agent WHERE id = $1;
+-- name: ArchiveAgent :one
+UPDATE agent SET archived_at = now(), archived_by = $2, updated_at = now()
+WHERE id = $1
+RETURNING *;
+
+-- name: RestoreAgent :one
+UPDATE agent SET archived_at = NULL, archived_by = NULL, updated_at = now()
+WHERE id = $1
+RETURNING *;
 
 -- name: ListAgentTasks :many
 SELECT * FROM agent_task_queue
@@ -55,6 +67,11 @@ UPDATE agent_task_queue
 SET status = 'cancelled'
 WHERE issue_id = $1 AND status IN ('queued', 'dispatched', 'running');
 
+-- name: CancelAgentTasksByAgent :exec
+UPDATE agent_task_queue
+SET status = 'cancelled'
+WHERE agent_id = $1 AND status IN ('queued', 'dispatched', 'running');
+
 -- name: GetAgentTask :one
 SELECT * FROM agent_task_queue
 WHERE id = $1;