feat: add gitleaks secret scanning setup and skill

- scripts/setup-gitleaks.sh — installs gitleaks v8.24.2, seeds
  .gitleaks.toml on first run, writes managed pre-commit hook block;
  re-run replaces block in place without disturbing other hook content
- scripts/gitleaks.toml — base config template extending default ruleset
- .gitleaks.toml — repo config with docs/research/ path allowlist
  (high-entropy terminal captures; v8.24.2 [allowlist] syntax)
- tests/test-setup-gitleaks.sh — 6 behavior tests including stale-block
  replacement and idempotency
- .agents/skills/gitleaks/ — cross-cutting skill covering install,
  update, allowlist tuning, scan modes, and real-finding remediation
- .agents/evals/cross-cutting/gitleaks/eval.yaml — 7 trigger + 3 output
  tests including version-aware allowlist guidance case
- docs/spec/overview.md — updated to reflect new tooling and skill

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.agents/evals/cross-cutting/gitleaks/eval.yaml

@@ -0,0 +1,84 @@
+skill_name: gitleaks
+
+trigger_tests:
+  - id: explicit-trigger-install
+    name: Explicit trigger — install and configure
+    query: "set up gitleaks in this repo"
+    should_trigger: true
+
+  - id: explicit-trigger-update-hook
+    name: Explicit trigger — update hook
+    query: "update the gitleaks pre-commit hook"
+    should_trigger: true
+
+  - id: implicit-trigger-false-positive
+    name: Implicit trigger — suppress false positive in pre-commit hook
+    query: "my pre-commit hook keeps blocking commits because it thinks my test fixture has an API key, how do I suppress it?"
+    should_trigger: true
+
+  - id: implicit-trigger-scan-history
+    name: Implicit trigger — audit repo history for secrets
+    query: "I want to scan my entire git history to make sure no credentials were ever committed"
+    should_trigger: true
+
+  - id: negative-trigger-security-review
+    name: Negative trigger — general code security review
+    query: "do a security review of this pull request"
+    should_trigger: false
+
+  - id: negative-trigger-ci-scan
+    name: Negative trigger — CI pipeline secret scanning
+    query: "add secret scanning to my GitHub Actions CI pipeline"
+    should_trigger: false
+
+  - id: negative-trigger-other-tool
+    name: Negative trigger — different secret scanning tool
+    query: "trufflehog is flagging false positives in my repo, how do I configure it?"
+    should_trigger: false
+
+output_tests:
+  - id: output-states-intent-before-running
+    name: Deterministic — states intent and waits for confirmation before running setup script
+    type: deterministic
+    prompt: "set up gitleaks in /home/user/myproject"
+    expected_output: >
+      The skill states what it is about to do (run setup-gitleaks.sh, which will modify
+      .git/hooks/pre-commit and seed .gitleaks.toml) and asks for confirmation before
+      executing the script. The confirmation request appears before any Bash command
+      that runs the script.
+    assertions:
+      - "Output contains 'setup-gitleaks.sh' before any Bash tool invocation that runs it"
+      - "Output contains a confirmation prompt (e.g. 'Proceed?') before running the setup script"
+      - "Output does not run the setup script without an explicit confirmation step"
+
+  - id: output-redacts-secret-values
+    name: Deterministic — redacts secret value and prescribes rotation before history rewrite
+    type: deterministic
+    prompt: "gitleaks found an AWS secret key on line 42 of config/settings.py — it says the value is AKIAIOSFODNN7EXAMPLE. What do I do?"
+    expected_output: >
+      The skill does not echo or repeat the secret value. It references the finding by
+      rule ID, file path, and line number only. It states that the secret must be rotated
+      immediately. It mentions git filter-repo (not git filter-branch) for history rewrite
+      and asks for user confirmation before running the rewrite.
+    assertions:
+      - "Output does not contain the literal string 'AKIAIOSFODNN7EXAMPLE'"
+      - "Output mentions rotating or revoking the credential as the first action"
+      - "Output references 'git filter-repo' for history rewrite, not 'git filter-branch'"
+      - "Output states that user confirmation is required before running the history rewrite"
+
+  - id: output-quality-allowlist-guidance
+    name: LLM-rubric — allowlist guidance is correct, version-aware, and minimal
+    type: llm-rubric
+    prompt: "gitleaks keeps flagging my docs/research/ directory as containing secrets, how do I suppress it?"
+    expected_output: >
+      High-quality output checks the installed gitleaks version before prescribing any
+      TOML syntax, recommends a path-based allowlist entry in .gitleaks.toml (not a
+      .gitleaksignore fingerprint), uses the correct TOML syntax for the detected version,
+      adds only the minimal allowlist entry needed for the identified false positive, and
+      includes a verification step (re-run gitleaks dir -v or gitleaks dir --log-level debug)
+      after making the change.
+    assertions:
+      - "Output checks or asks about the gitleaks version before writing TOML syntax"
+      - "Output recommends a path-based allowlist entry in .gitleaks.toml rather than .gitleaksignore"
+      - "Output includes a command to verify the suppression works after the change"
+      - "Output explains why .gitleaksignore fingerprints are fragile (line numbers shift)"