fix(agents): stop instructing agents to ToolSearch built-in tools (#167)

The system-prompt directive layer and the briefing block both opened
with "FIRST ACTION REQUIRED: run ToolSearch to activate deferred
Edit/Write". That premise is false: per Claude Code 2.1.114, ToolSearch
gates only deferred MCP tools, never built-ins — and it is not even a
callable tool in the agent runtime. Built-ins are loaded at spawn via
the `--tools` flag and gated solely by the per-role permission rules
(the actual Edit/Write breakage was the global Write(*)/Edit(*) deny +
single-slash path, fixed in c0ba335). So weak models dutifully chased a
nonexistent ToolSearch, concluded Edit/Write were unavailable, and
rewrote whole files via destructive shell redirection.

Both touch points now affirm the role's built-in tools are loaded and
ready, tell the agent NOT to call ToolSearch, and (for authoring roles)
explicitly steer away from whole-file shell redirection — directly
countering the clobber behaviour. Role prompt files (developer,
cell_pm, main_pm, board) updated to match. Dead
_read_tool_load_from_role_prompt (no callers) removed. Directive tests
rewritten to lock the corrected behaviour.

agents/prompts/roles/main_pm.md

@@ -12,7 +12,7 @@ You merge what your Cell PMs submit (cell PRs into your root branch via `complet
 
 - Your `task_id` (your root coordination task) and `agent_id` are pre-baked into the gateway session.
 - Your cell-PM slugs: `be-pm`, `fe-pm`, `ux-pm`. Your team: `board`. Your channel: `main-pm-board`.
-- Your verb manifest is loaded — MCP verbs are registered. Built-in tools (`Read`, `Bash`, `Task`, etc.) are deferred; run the `ToolSearch` call in the **First Action Required** block at the top of this prompt before your first read/bash.
+- Your verb manifest is loaded — MCP verbs are registered. Built-in tools (`Read`, `Bash`, `Task`, etc.) are loaded and ready — use them directly. Do NOT call `ToolSearch` (it does not gate built-in tools and is not available here).
 - Workspace: `/data/workspaces/{project}/board/main-pm/` — but you have no `Edit`/`Write` permission; this is just where merge operations resolve.
 
 ## Your verbs