feat: add issue search command

[schpetbot]

Summary

• add a first-class linear issue search <term> command backed by Linear's searchIssues API
• support team scoping, assignee/project/label/date filters, --include-comments, --include-archived, --order-by, --limit, and --json
• add command tests, a utility test for the tightened helper limit behavior, and README examples

Testing

• deno fmt src/utils/linear.ts test/utils/linear.test.ts README.md src/commands/issue/issue.ts src/commands/issue/issue-search.ts test/commands/issue/issue-search.test.ts test/commands/issue/snapshots/issue-search.test.ts.snap
• deno task check
• deno lint
• deno task test

Manual QA

• deno run --allow-all --quiet src/main.ts issue search "issue" --team CLI --limit 5
• deno run --allow-all --quiet src/main.ts issue search "issue" --team CLI --limit 3 --json
• deno run --allow-all --quiet src/main.ts issue search "issue" --team CLI --limit 3 --order-by updatedAt --json
• deno run --allow-all --quiet src/main.ts issue search "mark resolved" --team CLI --include-comments --json