feat(github): add multi-skill collection preview/apply

[santosli]

Summary

Implements Git-backed Skill Collections Phase C for v0.9.0 as a Draft/unreleased change set. Phase A+B remain shipped in v0.8.0; Phase D collection-level update/rollback remains planned.

• Adds Rust-owned GitHub repository/tree/ref collection preview and selected-child apply with one bounded fetch per operation.
• Reuses the schema-v9 collection/source/member provenance and stable collection identity from source plus explicit ref; preview identity binds resolved SHA, tree, children, and selection state.
• Adds structured Tauri/CLI outcomes for collection vs single-skill URLs and explicit-ref-required repository URLs; React does not route on human-readable errors.
• Rejects nested/overlapping skill roots, gitlinks, symlinks, unsafe paths/config/hooks/filters, stale trees, duplicate/conflicting children, and unsafe managed state before writes.
• Adds collection-level audited apply/rollback receipts, generation-gated close/reopen behavior, and desktop collection review UI.
• Keeps child deployment, Calls, and History independent; collection update/rollback remains Phase D.
• Updates public docs and keeps feat: Git-backed Skill Collections for v0.9.0 #46 open with Phase C in progress and Phase D planned.

Latest blocker follow-up

• Rollback receipts now bind each operation-owned managed target to no-follow filesystem kind/device/inode plus a bounded snapshot. Remote receipts also bind the current symlink, versions directory, and remote root. Replacements are preserved and reported as partial recovery rather than deleted.
• Both local and GitHub apply paths preflight the persisted collection reviewed SHA before operation/import/layout writes; the transactional persist_collection SHA check remains as defense in depth.
• Added regressions for same-content identity replacement during rollback and for a new child at a later remote SHA being rejected before any child/index/operation write.

Verification

• cargo fmt --check: passed.
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings: passed.
• cargo test --offline: CLI 31, core 354, desktop Rust 10, git 48, github 10; all passed.
• npm test: 275 passed; desktop build passed; npm audit --audit-level=high found 0 vulnerabilities.
• node site/build.mjs, node site/validate.mjs, and git diff --check: passed.
• cargo audit is externally blocked by the fetched RustSec database reporting duplicate advisory ID RUSTSEC-2026-0244; no dependency finding was reported.
• Browser public-preview Import Review remains checked at 1000x760 with no horizontal overflow; the latest commit changes core rollback/preflight and docs, not desktop layout.

Refs #46