Prevent exclusive-label conflicts and improve label diagnostics

[FelixLisczyk]

Problem

When users select multiple child labels from the same exclusive parent group, the CLI currently sends an invalid mutation to Linear. The resulting error is opaque, repeats the operation context, and may expose a truncated GraphQL mutation without identifying the conflicting labels or their shared group.

For example:

linear issues create "Issue title" --team SX --labels "Tests,Improvement"

The default label listings also render parent groups and child labels as a flat list, making invalid combinations difficult to recognize.

Changes

This PR:

• Resolves labels with canonical names, IDs, and parent metadata.
• Detects conflicting child-label selections before create, replace, and additive update mutations.
• Reports all conflicting labels and their shared exclusive group deterministically.
• Deduplicates label inputs and supports UUID and case-insensitive name resolution.
• Handles unchanged default teams correctly for add/remove label updates.
• Preserves explicit empty label updates for clearing all labels.
• Renders parent groups and child alternatives clearly in text label listings.
• Preserves the existing JSON label structure and parent metadata.
• Removes raw GraphQL query previews from user-facing errors.
• Removes duplicated create/update operation context.
• Adds resolver, validation, service, formatter, client, and GraphQL regression tests.

Testing

• make build passes.
• make test passes.
• Adds broad automated coverage for resolver/cache behavior, label conflict validation, mutation suppression, update modes, grouped formatting, JSON compatibility, and GraphQL error handling.
• Some planned edge cases do not yet have dedicated service or CLI integration tests, including full replace-conflict suppression and exact operation-context counts.