Feature: label management (create / delete / rename)

[muqsitnawaz]

What's missing

linear labels lists labels but has no CRUD:

$ linear labels --help
usage: linear labels [-h] [--json]

No way to:

• Create a label (labels create <name> [--color #hex])
• Delete a label (labels delete <id-or-name>)
• Rename a label (labels update <id-or-name> --name "...")

Why it matters

Labels accidentally created from truncated issue titles or one-off campaigns accumulate. Pasting a long title into Linear's quick-create dialog will sometimes turn part of the title into a label — examples from one workspace today:

• `d a QR code landing page`
• `sign-up flow for in-peron events (agents-cli.sh + get`
• `today`
• `qa-alpha.14`

There's no shell-friendly way to clean those up. Same use case as the cycles management proposal: weekly hygiene from the shell or a subagent without dropping to raw GraphQL.

Proposed surface

linear labels                                       # list (already works)
linear labels create <name> [--color "#hex"] [--description "..."]
linear labels update <id-or-name> --name "..." [--color "#hex"]
linear labels delete <id-or-name>

Accept fuzzy match on name.

Linear API reference

• Mutations: `issueLabelCreate`, `issueLabelUpdate`, `issueLabelDelete`