feat(dashboard): add hierarchical category taxonomy

[christso]

Summary

AgentV result categories now behave as slash-delimited analytics taxonomy paths, so security/network can be inspected as both a leaf and as part of the parent security rollup. Existing flat category labels remain valid, and explicit YAML category: values continue to override derived categories after canonicalization.

The Dashboard and results API now build category trees from loaded result metadata rather than persisted tree state. Parent drilldowns include descendant suites, leaf drilldowns stay precise, and compare score distributions filter by category metadata without parsing eval_path.

Category derivation is documented for generic eval filenames, named eval files, nested folders, and explicit overrides.

Validation

• bun run build
• bun test packages/core/test/evaluation/category.test.ts packages/core/test/evaluation/yaml-parser-metadata.test.ts apps/cli/test/commands/results/serve.test.ts apps/dashboard/src/lib/category-tree.test.ts apps/dashboard/src/lib/score-distribution.test.ts — 159 pass, 0 fail
• bun run typecheck
• bun run lint
• bun --filter @agentv/dashboard test — 147 pass, 0 fail
• bun --filter @agentv/core test — 2077 pass, 0 fail
• Browser/API UAT with a temporary run bundle under /tmp/agentv-cat-tree-uat: verified /categories exposes category_tree, parent security totals include security/network, /category/security shows both parent and descendant suites, /category/security%2Fnetwork shows only the leaf suite, and a narrow viewport renders the category table without obvious overlap.

Post-Deploy Monitoring & Validation

• This is local Dashboard/read-model behavior; no external production service monitoring is required.
• CI should cover build, typecheck, lint, and package tests for the stacked branch.
• After the base result-row sidecar work lands, run Dashboard against a real result bundle with nested categories and confirm parent rollups, leaf drilldown, and compare score distribution category filters still use category metadata only.

Risks

• This PR is stacked on result-row-id-sidecars / PR fix(results): isolate target bundles under run timestamps #1558 and should be merged or retargeted after that base lands.

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