Add `swamp extension test` command to run built-in extension tests with coverage

Problem

When working on extension models, there is no first-class command to run an extension's tests. The workaround requires knowing swamp's internal Deno binary path (~/.swamp/deno/deno), using the correct coverage output directory (coverage/) to match what CI expects, and running a separate deno coverage invocation to see the breakdown. This is friction that surfaces every time test coverage gaps need to be fixed.

Proposed Solution

Add a swamp extension test <package-or-path> command (and/or swamp model test <name>) that:

1. Locates the extension's *_test.ts files
2. Runs them using swamp's bundled Deno with the correct flags
3. Outputs the coverage table inline — branch %, function %, line % — without requiring a separate deno coverage invocation
4. Exits non-zero if any test fails or coverage drops below 100%

Alternatives Considered

• Documenting the raw deno test invocation in CLAUDE.md — works but puts the burden on every extension author to know swamp internals
• Using the CI workflow to check — slow feedback loop; requires a full workflow run just to see which lines are uncovered

Additional Context

Encountered while fixing a CI coverage failure on a PR. The only way to iterate on coverage was to manually invoke ~/.swamp/deno/deno test --coverage=coverage extensions/models/honeycomb/honeycomb_test.ts and then ~/.swamp/deno/deno coverage coverage/. A swamp extension test command would make this a one-liner and remove the need to know swamp's internal paths.