Add SnipCommand importer tool + move snips between CLIs

[StuartMeeks]

What

Implements the Importer backlog item from TODO.md: a cross-platform .NET console tool, snipdeck-importer, that brings command snippets in from SnipCommand — plus two supporting changes that fell naturally out of it.

1. tools/Snipdeck.Importer — the importer tool

• snipdeck-importer snipcommand <path> (Spectre.Console.Cli). Dry-run by default; --write is the only path that mutates a store.
• Options: --store, --write, --cli (force a CLI), --into (fallback CLI for unconfident suggestions), --allow-duplicates.
• On --write: backs the store up first (honouring the desktop app's retention), mints fresh GUIDs, creates CLIs on demand, and skips duplicates — then warns to restart Snipdeck.

Format finding: despite the .db extension, a SnipCommand export is pretty-printed JSON, not SQLite — so it's parsed with System.Text.Json (no new dependency). The ISnippetSource adapter still header-sniffs so a real-SQLite source could slot in later.

Pure, unit-tested pieces:

• ScMarkupTranslator — [sc_choice]/[sc_variable] → {token} + structured Parameter[]; slugifies names with spaces/punctuation ("Agreement ID" → AgreementID), disambiguates collisions, sanitises untrusted input, leaves malformed markup verbatim.
• CliSuggester — CLI from the first token, peeling launcher wrappers (sudo/npx) but not runtimes (pip).
• StoreMerger — pure Plan + Apply; CLI-scoped de-duplication by (CLI, Title, CommandTemplate).

2. Core: shared paths + tag parsing

• DefaultPaths centralises the data-path layout so the tool resolves the same store/backup locations as the app; WindowsPathProvider now delegates (behaviour unchanged).
• TagParser shares tag-CSV normalisation between the snip editor and the importer.

3. App: move a snip to a different CLI

The snip editor gains a CLI selector (the editor already received the CLI list — it was just unused). Re-homes a snip on save; pairs with the importer's --into fallback.

4. First-run seed

A snipdeck-importer CLI now sits alongside Examples, with snips demonstrating the importer's own commands.

Also

• Bumped Velopack 1.0.1 → 1.1.1 (the only stale package; the other 13 were already current).
• Added an Ubuntu importer-tests CI job.
• Docs: importer README + repo README pointer + CHANGELOG.

Testing

• Core: 213 tests pass. Importer: 47 tests pass (translator, suggester, merger, JSON source against a trimmed real-export fixture). Both run on Linux.
• E2E against a real SnipCommand export: dry-run lists 25 snips across mpt-app/pip/inv-app; --write creates the store + a backup; re-run skips all 25 as duplicates; --allow-duplicates re-imports.
• WinUI App head compile-verified on Windows (XAML CLI selector, interactions/path-provider changes, Velopack bump): 0 warnings, 0 errors.
• A medium-effort code review pass was run; its findings (uncaught retention crash, cross-CLI dedupe, dead CrossPlatformPathProvider, duplicated tag parsing) are addressed in this PR.

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