feat: add --add-labels and --remove-labels flags

[michael-ford]

Summary

The existing --labels flag on issues update uses replace semantics -- it sends a full labelIds array to the Linear API, wiping any existing labels on the issue. This makes it difficult to incrementally manage labels without knowing and re-specifying all current labels.

This PR adds two new flags:

• --add-labels -- adds the specified labels without removing existing ones
• --remove-labels -- removes the specified labels without affecting others

Both flags can be used together in a single command. They are mutually exclusive with --labels (which remains as the replace-all mode).

How it works

Since the Linear API's labelIds field is always a full replacement, the implementation:

1. Fetches the issue's current labels via the existing GetIssue query (which already returns labels { nodes { id name } })
2. Resolves the user-provided label names to UUIDs using the existing ResolveLabelIdentifier method
3. Merges (for --add-labels) or subtracts (for --remove-labels) using a set
4. Sends the combined label ID set via labelIds in the update mutation

No new GraphQL queries or API calls are needed beyond what already exists.

Usage examples

# Add labels without removing existing ones
linear issues update CEN-123 --add-labels "bug,urgent"

# Remove specific labels
linear issues update CEN-123 --remove-labels "wontfix"

# Add and remove labels in one command
linear issues update CEN-123 --add-labels "in-progress" --remove-labels "backlog"

# Replace all labels (existing behavior, unchanged)
linear issues update CEN-123 --labels "backend,security"

# Error: cannot mix replace with additive/subtractive
linear issues update CEN-123 --labels "foo" --add-labels "bar"
# => --labels cannot be combined with --add-labels or --remove-labels

Files changed

• internal/cli/issues.go -- new flag definitions, mutual exclusivity validation, updated help text and examples
• internal/service/issue.go -- AddLabelIDs/RemoveLabelIDs fields on UpdateIssueInput, merge/subtract logic, extractCurrentLabelIDs helper

Test plan

• Build succeeds: go build -o linear ./cmd/linear/
• All existing tests pass: go test ./... (396 tests)
• --add-labels and --remove-labels appear in linear issues update --help
• --labels with --add-labels returns an error
• --labels with --remove-labels returns an error
• --add-labels "newlabel" adds the label without removing existing labels
• --remove-labels "oldlabel" removes only the specified label
• --add-labels "a" --remove-labels "b" works together in one command
• --labels "a,b" still works as replace-all (existing behavior preserved)