Refactor Linear API token: pass through stack instead of setting env var

[acreeger]

Problem

LinearService constructor sets process.env.LINEAR_API_TOKEN as a side effect (src/lib/LinearService.ts:50), and all functions in src/utils/linear.ts rely on this env var being set via getLinearApiToken() → createLinearClient().

This caused a bug (#590) where il list --children called getLinearChildIssues without first instantiating LinearService, so the token was never in the environment.

Setting env vars as a side effect is fragile — any new code path that calls Linear utility functions without going through LinearService will hit the same UNAUTHORIZED error.

Desired behavior

All functions in src/utils/linear.ts should accept the API token as a parameter (with env var as fallback), passed down through the call stack. LinearService should stop setting process.env.LINEAR_API_TOKEN.

Scope

src/utils/linear.ts — Update all exported functions to accept an optional apiToken parameter and pass it to createLinearClient():

• fetchLinearIssue
• createLinearIssue
• createLinearChildIssue
• createLinearComment
• updateLinearIssueState
• getLinearComment
• updateLinearComment
• fetchLinearIssueComments
• getLinearChildIssues (already done in Refactor Linear API token: pass through stack instead of setting env var #590)
• createLinearIssueRelation
• getLinearIssueDependencies
• deleteLinearIssueRelation
• fetchLinearIssueList (already accepts apiToken)
• findLinearIssueRelation

src/lib/LinearService.ts — Stop setting process.env.LINEAR_API_TOKEN. Instead, store the token as an instance property and pass it to each linear.ts function call.

src/utils/image-processor.ts — Check if it reads LINEAR_API_TOKEN from env; if so, update to accept it as a parameter.

Tests — Update LinearService.test.ts to remove env var assertions. Test files that set the env var for test setup are fine.