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/internal/daemon/execenv/execenv.go

@@ -30,9 +30,10 @@ type PrepareParams struct {
 
 // TaskContextForEnv is the subset of task context used for writing context files.
 type TaskContextForEnv struct {
-	IssueID     string
-	AgentName   string
-	AgentSkills []SkillContextForEnv
+	IssueID           string
+	AgentName         string
+	AgentInstructions string // agent identity/persona instructions, injected into CLAUDE.md
+	AgentSkills       []SkillContextForEnv
 }
 
 // SkillContextForEnv represents a skill to be written into the execution environment.