feat(governance): Governance: PII Scan node with deterministic tealtiger guardrails

[CodeBhavyaG]

Closes #547

Summary

Adds a Governance: PII Scan plugin node that enforces deterministic
guardrails on graph execution, backed by the tealtiger package
(>=1.5.0, Apache-2.0) — no LLM calls, no network, sub-2.5ms per evaluation.

Node features

Config	Options
Mode	observe (fail-open, report only) / monitor / enforce
PII categories	SSN, Email, Phone, CreditCard (tealtiger) + APIKey (local regex)
PII action	detect (report only) / redact (typed tokens: [REDACTED_SSN]) / block

Outputs: output (transformed text), decision
({action: ALLOW|REDACT|DENY, findings[], risk_score, mode, detect_types, correlation_id, latency_ms}), blocked (boolean).

The decision object doubles as a per-execution audit record, per the
governance design in the issue. Observe mode never transforms or blocks;
tealtiger errors fail open and surface as a tealtiger-error finding
instead of crashing the graph.

Bugs hit along the way (and fixes)

1. TealGuard minimal config is a silent no-op: {pii: true, ...} flags
   don't exist on TealGuardConfig; a fresh TealGuard registers nothing.
   Fixed by registering PIIDetectionGuardrail explicitly.
2. TealGuard aggregation drops metadata: for pass-style actions
   (redact/mask) TealGuard.check() collapses the guardrail result to
   ALLOW / risk 0 and discards detections[]/redactedText. Fixed by
   wiring GuardrailEngine + PIIDetectionGuardrail directly.
3. Browser build: tealtiger's ESM bundle statically imports node
   builtins (fs, crypto) and unused provider SDKs whose trees are
   Node-only (aws-sdk, google genai, cohere). Fixed in the app via the
   import export condition, browser shims for fs/crypto (crypto-js
   based), and stubs for the unused SDKs. Note for tealtiger maintainers:
   an unknown detectTypes key failing silently is a footgun worth
   throwing on.

Files changed

• packages/core/src/plugins/governance/ — new plugin + GovernanceNode
• packages/core/src/plugins.ts, packages/app/src/plugins.ts — plugin registration
• packages/core/package.json — tealtiger@^1.5.0 dependency
• packages/app/vite.config.ts — ESM condition + browser aliases
• packages/app/src/shims/nodeBuiltins.ts — fs/crypto shims + provider stubs
• packages/app/package.json — crypto-js (shim)
• yarn.lock — lockfile

How to test

1. Unit tests

cd packages/core && yarn test

2. In the Rivet app

yarn dev   # vite dev app (the installed desktop app won't include this)

1. Plugins tab → add the Governance plugin
2. Build: Text → Governance: PII Scan → Output
3. On the node: Categories SSN, Email, CreditCard, Action Redact, Mode Enforce
4. Text: My SSN is 123-45-6789, email john@acme.com
5. Run → Output shows redacted text ([REDACTED_SSN]); Decision port shows {action: "REDACT", risk_score: ..., findings: [...]}
6. Switch Action to Block → output empty, blocked: true, action: "DENY"
7. Switch Mode to Observe → original text passes through, decision still reports findings

3. Latency

Each evaluation is pure regex: 0.2–2.5ms (measured), deterministic across runs.