Open Quickly: "On This Mac" Spotlight tier

[jedijashwa]

Spec: docs/specs/open-quickly-on-this-mac.md (merged in #34).

⌘K reaches only in-memory items today; a pasted relative link like docs/guide/setup.md matches nothing unless its folder is already open. The spec adds a Spotlight-backed secondary tier:

• "On This Mac" section below existing results — Markdown files + folders anywhere in the home directory; 5-row budget, expanding when tier 1 is empty.
• Matching: name predicate on the last path component (paths aren't queryable in Spotlight) + client-side whole-component path-suffix filtering for multi-segment fragments; ≥3-char activation gate; extension globs for md/markdown/mdown/mkd/mdx plus folders.
• Strictly secondary & jank-free: disk results never outrank tier 1, only append below, never move the selection; dedup against open files, the direct-path row, and recents; 150 ms debounce, one-shot home-scoped NSMetadataQuery off-main; hidden/node_modules/~/Library filtered.
• Opening: files land as ad-hoc opens; folders become sidebar roots (ties into Sidebar navigator: first-class navigation panel #32).
• Out-of-root link fix: clicking a relative link that escapes a document's resource root — today a silent no-op — opens ⌘K pre-filled with the path.
• Setting: "Search this Mac from Open Quickly" (General, default on).
• TCC posture: metadata queries never prompt; the standard prompt fires at file-open time, same as today's direct-path row.

Out of scope: Markdown content search / shipping an mdimporter, scope-config UI, non-Markdown results, index-health warnings. The implementing PR should say "Closes #". Implementation is a separate, separately-approved effort.

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