Add hosted pull request workspace and merge actions

[danielss-dev]

What changed

• Add a full-width Pull Requests workspace for GitHub and Azure DevOps repositories.
• Auto-open the active PR for the checked-out branch; keep the latest 100 PR index shallow and load rich details lazily.
• Render safe Markdown descriptions and comments, add top-level comments, show semantic check colors, and browse code changes through the Local Changes-style Pierre tree and diff view.
• Merge active PRs through a GitHub-style split button: run the selected strategy from the primary action or choose merge-commit, squash, or rebase from its keyboard-operable menu.
• Guard every merge with the exact loaded source commit while leaving required checks, reviews, queues, and branch policies provider-authoritative.
• Use the signed-in gh or az CLI without reading or storing provider tokens.

Why

Strand needs repository-scoped pull request review and merge workflows without forcing users into a browser or creating another credential store. Provider list/detail/diff calls remain separated so large repositories stay responsive and within query limits.

Validation

• cargo test -p strand-tauri pull_requests -- --nocapture
• cargo test -p strand-core (117 tests in a clean CI-like Git config)
• cargo clippy -p strand-core -p strand-tauri -- -D warnings
• pnpm --filter ./ui exec tsc --noEmit
• pnpm --filter ./ui test (208 tests)
• pnpm --filter ./ui build
• Live GitHub CLI validation against danielss-dev/strand

[danielss-dev]

does this comments work?