docs: Add Phase 1 CLI and Phase 2 API usage examples to README

[Copilot]

The README lacked concrete runnable examples for the CLI and REST API, leaving readers to infer invocation syntax from code and the API endpoint table alone.

Changes

Getting Started

• Replaced placeholder prerequisites/installation text with actual commands: pip install fastapi uvicorn pyyaml, and seeding ~/.notes/notes/ from test-notes/

Phase 1 — Running the CLI
New sub-section with labeled bash examples using the real entry point for every implemented command:

PYTHONPATH=src python3 -m notes_app.cli.main create "My First Note" "Content here."
PYTHONPATH=src python3 -m notes_app.cli.main list
PYTHONPATH=src python3 -m notes_app.cli.main read <note-id>
PYTHONPATH=src python3 -m notes_app.cli.main update <note-id> --tags "work,important"
PYTHONPATH=src python3 -m notes_app.cli.main delete <note-id>
PYTHONPATH=src python3 -m notes_app.cli.main search "keyword"

Phase 2 — Starting the API Server
New sub-section with uvicorn startup command and links to /docs, /redoc, and /health.

Phase 2 — Notes API usage
Labeled curl examples covering GET, POST, PATCH, PUT, DELETE, and /api/search?q=.

Phase 2 — Datasets API usage
Labeled curl examples for multipart CSV/JSON upload, list, metadata get/patch/delete, /preview, and /profile. Explicitly calls out the sidecar YAML creation and ~/.notes/datasets/ storage location.

API Endpoints table
Updated to reflect the actual implemented routes — PATCH added for both notes and datasets (previously only listed PUT).