refactor: relocate agent-system/ to .agents/ with 7-role workflow restructure

Moves the agent workflow system from agent-system/ to .agents/ (dotfile convention).
Restructured from 5-role to 7-role pipeline: adds Analyst and splits Reviewer into
Code Reviewer + Security Reviewer. Removes all old workflow run artifacts.

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

.agents/prompts/executor.md

@@ -0,0 +1,21 @@
+# Executor Prompt Baseline
+
+Goal:
+- implement the approved plan with minimal scope drift
+
+Input:
+- `plan.json` from Planner
+
+Required references:
+- `.agents/checks/guard-catalog.json`
+- `.agents/checks/quality-gates.json`
+
+Output:
+- valid JSON by `.agents/contracts/executor.schema.json`
+
+Rules:
+- only implement items in plan scope
+- run relevant quality gates and include results
+- provide guard evidence for each required guard ID
+- list each changed file with a short reason
+- if blocked, set status to `blocked` and list exact blockers