fix(rule): separate URI and metadata secret signals

[vsolano9]

Closes #9.

MCP030 now treats a resource URI as the strong signal: the complete sensitive-path pattern set still yields a critical finding there. Generic words in name or description no longer create a false critical, while unambiguous filename/path references such as .env, private-key filenames, and /etc/passwd remain visible as medium-confidence findings.

The rule description now states that confidence split, and regression coverage pins all three boundaries: negated generic prose, concrete metadata filenames, and sensitive URIs.

Validated locally:

• RED reproduced both the generic-prose false critical and the inability to downgrade concrete metadata evidence
• 53/53 tests; focused MCP030 coverage repeated 20 times
• lint, typecheck, and build
• complete tests/typecheck/build and built-CLI smoke on exact Node 20.20.2 and 22.23.2
• real static-manifest exit/severity checks for safe prose, medium metadata evidence, and a critical URI
• package dry-run includes the changed compiled rule
• git diff --check

Fresh production audit reports the same two existing transitive advisories; this PR changes no dependency or lockfile.