feat: add --label filter to issue list

[mihai-chiorean]

Summary

Adds -l, --label <label> flag to issue list for filtering issues by label name. The flag is repeatable — when multiple labels are specified, only issues matching all labels are returned.

• Uses eqIgnoreCase for case-insensitive label name matching
• Single label: filter.labels = { some: { name: { eqIgnoreCase: "Bug" } } }
• Multiple labels: filter.labels = { and: [{ some: { name: { eqIgnoreCase: "Bug" } } }, ...] }

Usage

# Single label
linear issue list --label Bug

# Multiple labels (AND logic)
linear issue list --label Bug --label "High Priority"

# Combined with other filters
linear issue list --label Bug --state started --cycle active

Test plan

• Added snapshot test for issue list --label Bug with mock server
• Updated help text snapshot with new --label flag
• deno fmt clean
• deno lint clean
• All tests pass, existing tests unaffected