feat(discovery): add Phase 1 shared discovery layer and discover CLI command

[yu-iskw]

Implements the discovery/query layer from the product Phase 1 strategy.

Changes:

• core/src/discovery/discovery-types.ts: New DiscoveryOutput schema with
  scored matches, confidence, reasons, disambiguation, related resources,
  and next_actions fields
• core/src/discovery/discovery-service.ts: DiscoveryService.query() — weighted
  ranking (exact/prefix/contains/tag/path/description/fuzzy), Levenshtein
  tolerance (edit distance ≤ 2), dependency centrality bonus, disambiguation
  for close candidates, depth-1 related resources, next-action suggestions
• core/src/discovery/discovery-service.test.ts: 12 fixture-based tests covering
  exact match, prefix, typo tolerance, type filter, no-match, disambiguation,
  related resources, next_actions, centrality bonus, inline token parsing
• core/src/index.ts + browser.ts: Export discovery types and DiscoveryService
• core/src/introspection/schema-generator.ts: Register discover command schema
• cli/src/discover-action.ts: New CLI action with human and JSON output
• cli/src/cli-actions.ts: Export discoverAction
• cli/src/cli.ts: Register dbt-tools discover "<query>" command with --type,
  --limit, --fields, --json, --no-json, --dbt-target flags

Quality gates: lint score 100, all 409 tests pass, coverage above thresholds,
knip clean.

https://claude.ai/code/session_01B9YSAvBqrwo9Ber77Dp7Zp