chore: upgrade knip to v6, add CI job

[jergason]

Summary

• Bump knip ^5.88.1 → ^6.0.3 (v6 uses oxc-parser instead of TS LanguageService — faster, no config changes needed for this repo)
• Add "knip": "knip" script to package.json
• Add knip job to pr-checks.yml github action (runs in parallel with lint, format-check, test)

Test plan

• pnpm knip passes clean
• All existing checks still pass (tests, lint, format, types)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Upgrades the repo’s dependency hygiene tooling by moving to knip v6 and adding a dedicated CI job so unused files/deps are caught during PR checks.

Changes:

• Bump knip to ^6.0.3 (and update related lockfile resolutions).
• Add pnpm knip as a package script.
• Add a knip job to the PR checks GitHub Actions workflow.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 3 comments.

File	Description
pnpm-lock.yaml	Updates lockfile to reflect knip v6 and associated dependency upgrades.
package.json	Adds knip script and bumps devDependency versions (including knip).
.github/workflows/pr-checks.yml	Introduces a new CI job to run pnpm knip.

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

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[Copilot]

knip@6 (and its oxc-parser dependency) declares engines.node: ^20.19.0 || >=22.12.0 in the lockfile. This job uses node-version: ${{ env.NODE_VERSION }} where NODE_VERSION is currently set to "20", which doesn’t guarantee a >=20.19.0 patch level and can cause pnpm install/pnpm knip to fail on engine checks. Consider pinning NODE_VERSION to 20.19.0+ (or 22.12.0+) or overriding the node version just for this job to match the new engine requirement.

[Copilot]

After upgrading to knip@^6.0.3, the lockfile indicates a stricter Node engine requirement (^20.19.0 || >=22.12.0). To prevent local installs (and other CI workflows) from breaking on older Node 20 patch versions, consider adding a top-level engines.node in package.json that matches the new requirement (or bumping the project’s documented Node version accordingly).

Suggested change

	},
	},
	"engines": {
	"node": "^20.19.0 || >=22.12.0"
	},

[Copilot]

PR description says knip is bumped from ^5.88.1, but package.json shows the previous specifier as ^5.86.0. If the description is meant to reflect the actual before/after, please update it (or adjust the dependency version if the intent was to start from ^5.88.1).