ci: harden Trunk tool discovery and YAML format

[KooshaPari]

Summary

• discover the Trunk Prettier tool directory from the action's INPUT_CACHE_PATH rather than assuming HOME or a .bin executable
• preserve plugin installation into Trunk's isolated tool directory
• satisfy yamllint comment spacing and line-length gates without disabling any linter

Evidence

• actionlint v1.7.8 targeted workflow
• frontend-rooted Prettier checks for changed YAML and frontend/web
• git diff --check

Stacked on PR #325; no merge performed.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[github-actions]

Legacy Tooling Scan Report

Severity	Count
Critical	0
High	0
Medium	0
Low	0

No violations detected.

This is a WARN-mode scan. Fix before strict enforcement begins.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

📊 Tier-2 Coverage Gate Results

Tier	Threshold	Status
Rust lib (byteport-transport)	≥71%	✅ Pass
Go framework (backend)	≥70%	✅ Pass
Frontend unit (frontend/web)	≥60%	✅ Pass