feat(issue): add --priority flag to lc issue create

[ruby-automation]

Summary

• Adds --priority PRIORITY option to lc issue create, accepting none, urgent, high, medium, low (case-insensitive), mapped to Linear integers 0–4
• Introduces LinearCli.CLI.Priority shared parser module (designed for reuse by EXT-65 lc issue update --priority)
• Priority is omitted from the GraphQL issueCreate mutation when not supplied; --output json includes priority metadata

Changes

• app/lib/linear_cli/cli/priority.ex — new shared priority name→integer parser
• app/lib/linear_cli/cli.ex — --priority option added to issue create subcommand spec
• app/lib/linear_cli/cli/commands/issues/create.ex — passes priority: through to creation opts
• app/lib/linear_cli/cli/issue/creation.ex — maybe_put_priority/2 omits field when nil
• app/lib/linear_cli/linear/issue.ex — :priority argument on :create action; maybe_put_priority/2 in GraphQL input builder
• documents/ash-domain-erd.adoc — updated create_issue row to reflect priority? field

Test plan

• mix test app/test/linear_cli/cli/priority_test.exs — 8 unit tests for the shared parser (all pass)
• mix test app/test/linear_cli/cli/commands/issues/create_test.exs — 6 new integration tests covering priority happy paths, nil omission, invalid value rejection, case-insensitivity, JSON output
• mix test app/test/linear_cli/linear/issue_test.exs — 2 new tests confirming GraphQL input includes/omits "priority" correctly
• Full suite: 550 passed, 0 failures

Closes EXT-66

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