chore(release): prepare v0.12.0

[Microck]

Summary

• Bump Rust CLI and npm wrapper metadata to 0.12.0
• Add CHANGELOG.md notes for the assistant contract, JSON error, named lens, and extract filter features
• Update docs footer and release runbook details
• Make release workflow warnings explicit when optional package/docs sync credentials are missing

Verification

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -q --locked
• (cd npm && npm pack --dry-run)

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Project	Status	Preview	Updated (UTC)
kagi-cli	🟡 Building	–	Jun 28, 2026, 10:15 PM

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