feat(app): add PR inbox with LLM summaries

[vitaliytv]

What changed

• adds a global PR Inbox backed by the locally authenticated gh CLI
• groups open PRs by required author action, requested review, assignment, or waiting on others
• adds on-demand LLM summaries through the configured ACP agent
• treats GitHub PR body/comments as untrusted context and prohibits agent tool calls in the summary prompt

Verification

• cargo test -p task --lib pull_requests
• bunx vitest run src/tests/pull-requests-dialog.test.js src/tests/pipeline-status-dialog.test.js
• bun run build
• npx @7n/rules lint doc-files --no-fix --path app/src/components
• npx @7n/rules lint doc-files --no-fix --path app/src-tauri/src
• npx @7n/rules lint changelog