feat: pixel-art agent-style tag dots

[jonnydry]

Summary

Adds a TagDot component that renders pixel-art "agent dot" indicators for tags across the UI, replacing the previous solid-color dots. Rebased onto current main.

• New src/components/tag-dot.tsx — sprite-based indicator (256×256 sheet, 8×8 grid) with imageRendering: pixelated, tuned for tiny sizes (6–14px).
• New getTagDotVariant helper in src/lib/tag-colors.ts (stable hash → variant).
• New sprite sheet public/tag-icons/pixel-agent-dots.png.
• Swaps solid dots → TagDot in: sidebar, settings tag rows, command palette, orbit map graph search, orbit map rail, orbit review fields, orbit scan overview strip, share views.

Rebase notes

This branch was originally cut from an older main; it has been rebased onto the current tip. During the rebase, main's structural refactors were preserved and the tag-dot swaps were re-applied to their new locations:

• The orbit map page was extracted into orbit-map-client / orbit-map-graph-search — the tag search-result dot was re-applied there.
• orbit-scan-overview-strip now uses a TagRollupChip component; the dot swap was applied inside it.
• orbit-review-fields now uses LibrarySearchPicker; the picker item dot was swapped.
• Two components the feature originally modified (library-bookmark-inspector, orbit-slide-in-panel) were deleted on main and were accepted as deleted.
• Kept main's design-contract-compliant styling (surface utilities, ring-hairline-soft) rather than the branch's older ad-hoc rings.

An unrelated, stale security-hardening diff that had been sitting uncommitted in the source worktree was intentionally not included — that work already landed on main (via chore/security-hardening-and-review-fixes) in a newer form; including it would have reverted the 1d7a3d6 P0 fixes.

Verification

• tsc --noEmit ✅
• eslint on changed files ✅ (design-language contract is ESLint-enforced)
• npm run build ✅

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