perf: pre-allocate string in compact_context

[bug-ops]

Summary

• Replace format! + collect::<Vec<_>>().join() with String::with_capacity() + std::fmt::Write in compact_context
• Eliminates intermediate string allocations per message

Closes #316

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-core/src/agent/context.rs	92.30%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main     #329      +/-   ##
==========================================
- Coverage   80.37%   80.36%   -0.01%     
==========================================
  Files          99       99              
  Lines       24163    24166       +3     
==========================================
+ Hits        19421    19422       +1     
- Misses       4742     4744       +2     

Files with missing lines	Coverage Δ
crates/zeph-core/src/agent/context.rs	85.91% <92.30%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes

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