✨ feat(fermata): add secret filtering engine — the security brain

Implement Goals 1–3 and 5 from the reveal-layer security brain goal.
fermata now detects, redacts, and scans for secrets in AI agent tool
output, filling the ecosystem gap where no coding agent filters secrets
post-read.

New core/secrets/ module:
- config.rs: .botsecrets TOML format with hierarchical merge and ~40
  built-in key patterns
- parser.rs: multi-format secret file parser (.env, TOML, YAML, JSON,
  Python assignments, Java properties)
- manifest.rs: file discovery + parsing → known-secrets set
- redactor.rs: Aho-Corasick multi-pattern replacement with 4 styles
- scanner.rs: RegexSet heuristic detection with 35 gitleaks-derived
  patterns (MIT) and Shannon entropy filtering
- patterns.rs: curated rules for AWS, GitHub, Stripe, Slack, JWT, etc.

Hook integration:
- fermata hook --event post-tool-use reads tool output, runs redactor +
  scanner, returns updatedToolOutput for Claude Code
- Backward compatible: --event pre-tool-use (default) unchanged
- Fail-open: errors produce {} and exit 0

Library API:
- Redactor::new(manifest, style).redact(text) → RedactedText
- Scanner::new(config).scan(text) → Vec<Finding>
- Compiles without CLI feature for embedding in other crates

195 tests (130 new), all passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/core/secrets/mod.rs

@@ -0,0 +1,15 @@
+//! Secret-filtering configuration (`.botsecrets` files), multi-format
+//! secret file parsing, and heuristic scanning.
+
+pub mod config;
+pub mod manifest;
+pub mod parser;
+pub mod patterns;
+pub mod redactor;
+pub mod scanner;
+
+pub use config::SecretsConfig;
+pub use manifest::{Manifest, ManifestError};
+pub use parser::{parse_secret_file, FileFormat, ParseError, SecretEntry};
+pub use redactor::{RedactedText, Redaction, Redactor};
+pub use scanner::{Confidence, Finding, Scanner};