feat(tui): scope notes to projects

[HonestMajority]

Summary

• Store notes per project under ~/.agman/projects/<project>/notes/ via Config::project_notes_dir.
• Reuse the existing notes explorer/editor with a project-specific root from ProjectList and TaskList.
• Add caller-aware notes close behavior so notes return to ProjectList or TaskList, and keep legacy global ~/.agman/notes untouched.

UX

• ProjectList o opens notes for the selected real project.
• (unassigned) shows a status and does not open/create shared notes.
• TaskList uppercase O opens current-project notes; lowercase o still opens linked PRs.

Validation

• Researcher agman-improvements--project-scoped-notes approved with no blockers.
• cargo test
• cargo clippy --all-targets -- -D warnings
• git diff --check