Make BMAD dependency preflight read-only

[pdurlej]

Summary\n\n- replace the stateful BMAD dependency invocation with a local read-only structural preflight\n- keep release/reference drift and state-cache write failures nonblocking\n- stop suggesting automatic BMAD check/sync remediation during task startup\n- verify installer behavior through the read-only X3 gate\n\n## Why\n\nThe previous compact gate invoked BMAD's sync helper even for a health check. In sandboxed Codex sessions that helper could write under ~/.codex, fail with PermissionError, and make X3/X4 appear unhealthy. That created unnecessary sync prompts and blocked normal work.\n\nThe new gate checks the installed local capability directly. Missing or structurally unusable BMAD files still fail closed. Update freshness remains advisory.\n\n## Verification\n\n- 107 deterministic tests passed using the same commands as main CI\n- X1, X2, X3, and X4 smoke checks all allow execution\n- X3 and X4 return bmad_status=ok with no remediation\n- the regression suite proves an unwritable BMADX state cache does not block X3 or suggest synchronization\n- git diff --check and Python byte-compilation passed\n\n## Scope\n\nThis PR intentionally excludes the unfinished value-study benchmark and its performance claims.