feat(cli): add --tag / --exclude-tag filtering for eval runs

[christso]

Problem

Eval files support tags: [...] in the schema, but the CLI has no way to filter by them. CI currently uses hardcoded glob patterns to exclude multi-provider evals, which is fragile.

Proposal

Add --tag and --exclude-tag flags to agentv eval run:

# Run only agent-tagged evals
agentv eval run 'examples/**/*.eval.yaml' --tag agent

# Run everything except multi-provider evals
agentv eval run 'examples/**/*.eval.yaml' --exclude-tag multi-provider

# Combine (AND logic, matching promptfoo convention)
agentv eval run 'examples/**/*.eval.yaml' --tag agent --exclude-tag slow

Prior art

• promptfoo: --filter-metadata tags=X on test cases
• pytest: @pytest.mark.slow + -m "not slow" marker filtering
• Jest: --testPathPattern + --testNamePattern for filtering

Both promptfoo and pytest support inclusive and exclusive filtering with AND logic for multiple filters.

Implementation notes

• Tags are already parsed into EvalFileMetadata.tags (packages/core/src/evaluation/metadata.ts)
• Filter should apply at the eval-file level (skip entire files), not test-case level
• --exclude-tag would replace hardcoded glob exclusions in CI workflows