feat(policy): add MCP policy layer with startup validation

[ghul0]

Problem

The adapter currently exposes all tools and resources from a configured MCP server and forwards tool arguments without an adapter-side policy layer. That makes it hard to enforce allowlists and request-shaping rules for higher-risk servers.

Root Cause

Server definitions only described connection and exposure settings. There was no config-level policy model to:

• limit which tools/resources remain visible
• reject disallowed tool calls before they reach the MCP server
• inject or validate required request fields consistently at adapter startup and call time

Fix

• add a per-server policy model with allowlists for tools, resources, and prompts
• add per-tool request rules for defaults, required keys, forbidden keys, allowed values, and per-item validation/injection
• validate policy config fail-fast during adapter initialization
• filter visible tools/resources and intersect direct-tool exposure with policy allowlists
• enforce policy at MCP call time before dispatching requests
• include policy in metadata cache hashing so cached metadata reflects policy changes

Summary

• add new policy.ts implementation and ServerEntry.policy typing
• extend config validation with validatePoliciesInConfig()
• apply policy filtering in metadata/direct-tool paths
• apply policy enforcement in proxy call execution
• document the feature in README.md
• add focused policy tests only

Testing

Policy test suite

npx vitest run __tests__/policy.test.ts __tests__/policy-apply.test.ts __tests__/policy-allowlist.test.ts __tests__/policy-cache-warnings.test.ts __tests__/policy-init-wiring.test.ts __tests__/policy-integration.test.ts

 RUN  v3.2.4 /tmp/pi-mcp-adapter-pr2

 ✓ __tests__/policy.test.ts (24 tests) 5ms
 ✓ __tests__/policy-allowlist.test.ts (24 tests) 5ms
 ✓ __tests__/policy-apply.test.ts (28 tests) 6ms
 ✓ __tests__/policy-cache-warnings.test.ts (11 tests) 7ms
 ✓ __tests__/policy-integration.test.ts (12 tests) 8ms
 ✓ __tests__/policy-init-wiring.test.ts (1 test) 3ms

 Test Files  6 passed (6)
      Tests  100 passed (100)
   Duration  386ms

Standalone note

This PR is intentionally based directly on upstream/main and does not include the repeated --mcp-config work from PR #29.

The only startup wiring change here is the standalone-compatible init.ts addition of:

• validatePoliciesInConfig(config)

while preserving upstream's existing single-config loading:

• const configPath = pi.getFlag("mcp-config") as string | undefined;
• const config = loadMcpConfig(configPath);