session-handoff validator rejects valid ### headings in required sections

Summary

skills/session-handoff/scripts/validate_handoff.py currently treats required sections as present only when their headings use # or ##.

That causes valid handoff documents using deeper Markdown headings such as:

• ### Immediate Next Steps
• ### Important Context

to be reported as incomplete, even though the section names and content are correct.

Reproduction

Minimal handoff fragment:

# Handoff: Example

## Current State Summary

This is a valid summary with enough content to pass the completeness check.

### Immediate Next Steps

1. Do the first thing.
2. Do the second thing.
3. Do the third thing.

### Important Context

This is the critical context the next agent must know before resuming work.

Current validator result:

• marks Immediate Next Steps as missing
• marks Important Context as missing

Expected result:

• both sections should be recognized as valid required sections

Root Cause

The validator currently matches headings with a regex equivalent to:

pattern = rf'(?:^|\\n)##?\\s*{re.escape(section)}'

That only accepts one or two # characters.

Proposed Fix

Allow any Markdown heading depth:

pattern = rf'(?:^|\\n)#+\\s*{re.escape(section)}'

The same adjustment should be applied consistently to:

• required section detection
• recommended section detection

Why This Matters

The current behavior creates false negatives in otherwise valid handoffs and forces users/agents to rewrite heading levels just to satisfy the validator.

This is especially confusing because the semantic structure is correct; only the heading depth differs.

Scope

This should be a small, low-risk bugfix:

• expand heading-level matching
• add a regression test for ### headings
• keep the rest of validation logic unchanged