Add inline pull request review threads

[danielss-dev]

Summary

• load GitHub review threads separately from top-level PR comments, preserving file ranges, replies, and resolved/outdated state
• render persistent inline thread cards in the Pierre Changes diff and include review comments in Conversation
• use Pierre’s native hover-gutter comment affordance and add a keyboard-accessible View in changes action from file-backed timeline comments
• update the roadmap, task ledger, README, and pull request guide

Why

GitHub review comments are stored in the review-thread connection rather than the issue-comment collection Strand previously loaded. Comments created successfully on GitHub therefore disappeared from Strand after refresh and could not be navigated from Conversation back to their code.

Impact

Review comments now remain visible inside Strand, replies stay grouped at their code location, and reviewers can jump directly from Conversation to the corresponding file and inline thread.

Validation

• pnpm --filter ./ui test — 217 tests passed
• pnpm --filter ./ui exec tsc --noEmit
• cargo test -p strand-tauri --bin strand pull_requests — 9 tests passed
• cargo check -p strand-core -p strand-tauri
• git diff --check