fix(yaml): quote leading-zero numeric strings

[fibibot]

Summary

• Quote leading-zero numeric-looking strings in YAML stringify output so values like 08 and 09 are not emitted as plain scalars.
• Add regression coverage for 00, 07, 08, 09, 010, and -08.

Closes bartlomieju/orchid-inbox#73

Testing

• deno fmt --check
• deno lint (passed with existing workspace-member warning for @std/crypto)
• deno test yaml/stringify_test.ts
• deno test -A --trace-leaks --coverage --doc --clean yaml

deno task test was attempted, but the VM killed the process with exit 137 during the parallel full suite before any failure was reported. A full non-parallel equivalent was also attempted and stopped at the VM level before reporting a test failure.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (5ea9159) to head (72d1754).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7144   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files         634      634           
  Lines       51830    51847   +17     
  Branches     9341     9347    +6     
=======================================
+ Hits        49037    49054   +17     
  Misses       2218     2218           
  Partials      575      575           

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