fix(linker): link scenarios by exact @id / @scenario overlap only

[vivganes]

Closes #21

Problem

Linking currently used any shared tag overlap, so category/filter tags like @smoke or @P1 unintentionally linked one test to many scenarios. This inflated completion stats and produced noisy "100% coverage" claims.

Solution

Introduce explicit, namespaced linking tags:

• Gherkin scenario: @id:<value>
• Test result: @scenario:<value>

Linking now matches only on exact ID overlap. Category tags remain in parsed output for display/reporting, but no longer participate in scenario-to-test linkage.

Behavior change

• Before: any(tag in scenario.tags for tag in result.tags)
• After: exact match between scenario @id:* values and result @scenario:* values

Verification

• Updated existing fixtures/tests to use the new conventions.
• Added regression tests for exact-match linking and shared-tag isolation.
• Full suite passes: 139/139.