feat(guardrails): write the identity-domain map from the machine profile in setup #9749

Parent

Refs #5945

Depends on

• feat(guardrails): add the identity-domain guard with domain classification and its kill switch #9743

Outcome

/guardrails:setup offers to write the guard's domain map from /harness-ops:machine-profile apply: each
recorded identity domain's tree becomes its roots and its gh directory its gh_config_dir. Values discovery
does not record (owners, identity markers, az_config_dir, shared exceptions) are asked of the operator or left
empty. The map is written only to the plugin data directory, after the operator confirms.

Decisions

• Epic feat(guardrails): generalize machine-local safety guards into plugin guards #5943 defaults, adopted: the guard lives in guardrails behind its dispatcher and a per-guard kill switch;
  bash under the node entry point; port behavior, not the local Python.
• The domain map is derived from /harness-ops:machine-profile and is machine-local: it never sits in a tracked
  file, fixture or doc. Placeholders only.
• Fail open on any internal error; every exit is 0 with the decision in stdout JSON.
• The guard reads command text: a deterrent, not a sandbox. Declared gaps: computed paths, scripts run by name.

Key interfaces

• plugins/guardrails/hooks/block-cross-domain.sh and plugins/guardrails/hooks/block-cross-domain.test.sh, the Bash|PowerShell row of plugins/guardrails/hooks/hooks.json,
  plugins/guardrails/README.md, docs/conventions/hook-precision, docs/conventions/hook-budget.

Done when

• A setup test feeds a machine-profile fixture with two domains and finds a map with both domains' roots and gh_config_dir in a temp CLAUDE_PLUGIN_DATA.
• The test asserts no file under the repository checkout changed.
• grep -n "identity-domains" plugins/guardrails/skills/setup/SKILL.md prints the step.
• git grep -nE "melodic|kyle|sexton" -- plugins/guardrails/hooks/block-cross-domain.sh plugins/guardrails/hooks/block-cross-domain.test.sh prints nothing (placeholders only).
• scripts/run-test-file.sh plugins/guardrails/hooks/run-guards.test.sh exits 0, including the guardrails: Bash|PowerShell PreToolUse row takes 30+s on a ~70 KB command; its 60s timeout fails open #4528 long-command ceiling case.
• The PR reports the Bash|PowerShell row's added time, measured as docs/conventions/hook-budget describes, inside that budget.
• scripts/check-changelog-fragments.sh --check-required origin/main exits 0.