feat: enforce functional verification, mandatory quality pipeline, and requirement checks

Add three new sections to AGENTS.md addressing workflow gaps observed in
a session where 6 features were implemented but none were functionally
tested — only static analysis (type checks, linting) was used, resulting
in broken features shipped as 'done'.

New rules:
- Functional Verification: features must be end-to-end tested before
  completion; static analysis alone is explicitly insufficient
- Mandatory Quality Pipeline: every feature gets reviewer + tester passes;
  no batch validation; quality over quantity under time pressure
- Requirement Understanding Verification: verify understanding before
  implementing features with implicit expectations or domain concepts

Also simplifies tester bash permissions and adds plan write permission.

.config/opencode/opencode.jsonc

@@ -12,7 +12,8 @@
 		},
 		"explore": {
 			"disable": true
-		}
+		},
+		"plan": { "permission": { "write": "allow" } }
 	},
 	"permission": {
 		"websearch": "allow",