[Midtown !3] Wire expand_session_id_in_prompt into spawn

[btucker]

Summary

• Calls expand_session_id_in_prompt() on headless_config.system_prompt immediately after the UUID is pre-assigned at spawn time
• Calls expand_session_id_in_prompt() on the initial prompt before delivering it via stdin
• Fixes the root cause: single-quoted heredocs (<<'EOF') used when passing prompts to the claude CLI suppress shell variable expansion, so $MIDTOWN_SESSION_ID was never replaced — agents were writing the literal placeholder into GitHub frontmatter instead of the real UUID

Test plan

• Existing test_expand_session_id_replaces_env_var_in_prompt and test_reviewer_system_prompt_contains_unexpanded_session_id tests cover the expansion function itself
• cargo test spawn passes
• cargo test expand_session_id passes
• Spawned agents now embed the real session UUID in <!-- midtown session:UUID --> PR/comment frontmatter

🌃 Co-built with Midtown

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.95%. Comparing base (05897b0) to head (165f5c2).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/daemon_v2/executor/spawn.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2334      +/-   ##
==========================================
+ Coverage   63.70%   63.95%   +0.25%     
==========================================
  Files         100      100              
  Lines       38582    38596      +14     
==========================================
+ Hits        24578    24684     +106     
+ Misses      14004    13912      -92     

Files with missing lines	Coverage Δ
src/daemon_v2/executor/spawn.rs	97.91% <85.71%> (+0.09%)	⬆️

... and 8 files with indirect coverage changes

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