[M2][PR2.1] Rewrite practice_plan.md around action

[kymuco]

Goal

Make practice_plan.md read like a concrete next-practice plan instead of a generic analysis artifact.

The user should immediately understand what to keep, what to fix first, and what to record next.

Scope

Update Markdown wording/structure only.

Do not change:

• scoring;
• alignment;
• audio preprocessing;
• JSON payload contracts;
• CLI/API behavior;
• generated artifact filenames.

Suggested structure

Add or clarify sections such as:

What to keep
What to fix first
Why this matters
Next recording target

Acceptance

• practice_plan.md starts with action-oriented guidance.
• The strongest stable area is easy to find.
• The primary weakness is easy to find.
• The next recording target is concrete and musician-facing.
• Existing tests are updated or new Markdown tests are added.
• No core analysis behavior changes.