feat(agent): add instructions field for agent persona/identity

Add an `instructions` text field to the agent model, allowing users to
define each agent's role, expertise, and working style. Instructions are
injected into CLAUDE.md as an "Agent Identity" section so the agent
knows who it is on every task execution.

- Migration 021: add instructions column to agent table
- Backend: create/update/get agent handlers support instructions
- ClaimTask response includes instructions for daemon injection
- execenv: inject instructions into CLAUDE.md meta-skill
- Frontend: add Instructions tab to agent detail panel

server/pkg/db/queries/agent.sql

@@ -11,8 +11,8 @@ WHERE id = $1;
 INSERT INTO agent (
     workspace_id, name, description, avatar_url, runtime_mode,
     runtime_config, runtime_id, visibility, max_concurrent_tasks, owner_id,
-    tools, triggers
-) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12)
+    tools, triggers, instructions
+) VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13)
 RETURNING *;
 
 -- name: UpdateAgent :one
@@ -28,6 +28,7 @@ UPDATE agent SET
     max_concurrent_tasks = COALESCE(sqlc.narg('max_concurrent_tasks'), max_concurrent_tasks),
     tools = COALESCE(sqlc.narg('tools'), tools),
     triggers = COALESCE(sqlc.narg('triggers'), triggers),
+    instructions = COALESCE(sqlc.narg('instructions'), instructions),
     updated_at = now()
 WHERE id = $1
 RETURNING *;
@@ -96,6 +97,11 @@ RETURNING *;
 SELECT count(*) FROM agent_task_queue
 WHERE agent_id = $1 AND status IN ('dispatched', 'running');
 
+-- name: HasActiveTaskForIssue :one
+-- Returns true if there is any queued, dispatched, or running task for the issue.
+SELECT count(*) > 0 AS has_active FROM agent_task_queue
+WHERE issue_id = $1 AND status IN ('queued', 'dispatched', 'running');
+
 -- name: ListPendingTasksByRuntime :many
 SELECT * FROM agent_task_queue
 WHERE runtime_id = $1 AND status IN ('queued', 'dispatched')