feat: support comma-separated values in --state flag

[jholm117]

Summary

• Allow --state triage,started as a shorthand for --state triage --state started
• Comma-separated values are split, trimmed, and validated against valid states (triage, backlog, unstarted, started, completed, canceled)
• Invalid values produce a clear ValidationError with the list of valid states

Motivation

Passing --state triage,started is the intuitive way to specify multiple states in a single flag, but previously only the repeated flag form (--state triage --state started) worked. This change supports both forms.

Changes

• src/commands/issue/issue-list.ts — Changed --state flag type from state (EnumType) to string so comma-separated values pass through Cliffy parsing. Added flatMap + split(",") to parse comma-separated values, with manual validation against validStates.
• test/commands/issue/issue-list.test.ts — Added snapshot tests for invalid comma-separated state and invalid single state validation errors.
• test/commands/issue/__snapshots__/issue-list.test.ts.snap — Updated help text snapshot (new description) and added error output snapshots.

Test plan

• deno test --allow-all --quiet test/commands/issue/issue-list.test.ts — 3/3 pass
• deno check src/commands/issue/issue-list.ts — clean
• deno lint src/commands/issue/issue-list.ts — clean
• Manual test: linear issue list --state triage,started --sort priority
• Manual test: linear issue list --state triage --state started --sort priority (existing behavior preserved)

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