feat: match --test filters against the full describe-path

[kevinccbsg]

Summary

• --test / testNames filters now match a case-insensitive substring of the full describe-path ("Describe > nested > test name") instead of only the it() name — mirroring twd-cli's semantics, so filtering by a describe name runs all tests under it (and cross-boundary filters like "login flow > shows error" work).
• New pure helpers src/browser/buildTestPath.ts and src/browser/filterTests.ts (ported from twd-cli), wired into handleRunCommand.
• NO_MATCH error now lists available tests as full describe-paths so agents can construct a working retry.
• README updated; design spec included under docs/superpowers/specs/.

Why

AI agents (and developers) assume describe-name filtering is default behavior, as in Vitest/Jest — the name-only match was a recurring source of friction with agent-driven runs.

Testing

• 12 new unit tests for path building and filtering; full suite 85/85 passing, build clean.

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