feat: OS-level quick capture (context menu / Share extension) for Desktop platforms

[tstapler]

Problem

Android has a fast capture path: share-sheet → CaptureActivity → straight into today's journal (see androidApp/src/main/kotlin/dev/stapler/stelekit/CaptureActivity.kt). Desktop has no equivalent — there's no way to select text/a file anywhere in the OS and send it into SteleKit without switching to the app and using in-app Import.

This is distinct from #230 (Chrome web-clipper extension, browser-scoped) and #232 (local REST API): this issue is about the OS capture surfaces outside the browser — right-click "Send to SteleKit" from a file manager, macOS Services menu / Share extension, Windows context-menu handler, or a global hotkey capture popup — mirroring what CaptureActivity already gives Android users, for Desktop.

Why this matters

• Closes the capture-friction gap between mobile and desktop; today desktop users have strictly more friction to add a quick note than Android users.
• Once feat: local REST/automation API (ETAPI-equivalent) #232's local API exists, this is a thin OS-integration client of it (same relationship the web clipper in feat: web clipper browser extension #230 has) — not a new write path.

Suggested scope (v1)

• macOS: a Share Extension / Services-menu entry ("Add to SteleKit") that takes selected text and posts it via the local API from feat: local REST/automation API (ETAPI-equivalent) #232, landing in today's journal — same target behavior as Android's CaptureActivity.
• Linux: a file-manager "Send to" script / Nautilus action for text files or a global-hotkey capture popup (desktop environment conventions vary — scope to whichever is lowest-effort first, e.g. a small always-running capture window bound to a hotkey).
• Windows: context-menu registry handler invoking the same local API endpoint.
• Reuse the same enrichment pipeline being wired into Android capture (see companion issue "auto-link + tag-suggest content captured via Android share sheet") once it lands, so desktop quick-capture gets the same auto-linking/tag-suggestion, not a second bespoke path.

Out of scope (v1)

• Full Import UI parity (scope selection, format picker) — this is the quick capture path, equivalent to Android's minimal bottom sheet, not the in-app Import screen.

References

• androidApp/src/main/kotlin/dev/stapler/stelekit/CaptureActivity.kt — Android reference implementation
• Depends on: local REST/automation API (feat: local REST/automation API (ETAPI-equivalent) #232)
• Related, browser-scoped only: web clipper browser extension (feat: web clipper browser extension #230)