chore(release): prepare 0.6.7

[leehack]

Summary

• bump package and docs references from 0.6.6 to 0.6.7 for the upcoming release
• add 0.6.7 release notes covering the b8373 native sync, Linux loader hardening, and the Hermes tool-call parsing fix
• keep the tag-driven publish and docs workflows aligned with the release branch metadata

Validation

• dart format --output=none --set-exit-if-changed .
• dart analyze
• ./tool/docs/validate_links.sh
• dart test -p vm -j 1 --exclude-tags local-only

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.90%. Comparing base (5f8e0e3) to head (4371725).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #90   +/-   ##
=======================================
  Coverage   76.90%   76.90%           
=======================================
  Files          67       67           
  Lines        8368     8368           
=======================================
  Hits         6435     6435           
  Misses       1933     1933           

Flag	Coverage Δ
unittests	76.90% <ø> (ø)

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