feat(issue): add interactive issue assign command

[bougyman]

Summary

• Adds lc issue assign <issue-id> — assigns an issue to any team member, not just self (which issue take already handles)
• Fetches assignable members from the issue's resolved team via team.members GraphQL query (confirmed in schema)
• Follows the issue status pattern exactly: --assignee/-a flag for non-interactive use, Prompt.select/2 when omitted
• Name resolution: case-insensitive exact match, then unique prefix match (same algorithm as workflow state resolution)
• Alias: lc issue a CRY-1 routes to issue assign

Changes

• app/lib/linear_cli/linear/user.ex: New User.Read.ByTeam manual read action + :by_team action declaration
• app/lib/linear_cli/linear.ex: Linear.team_members/1 domain code interface
• app/lib/linear_cli/cli/commands.ex: Commands.issue_assign/1 command function with member resolution helpers
• app/lib/linear_cli/cli.ex: Optimus spec, dispatch clause, "a" alias

Test plan

• --assignee exact name match (case-insensitive)
• --assignee unique prefix match
• --assignee unknown name → exits 22 with "smells bad" message
• --assignee ambiguous prefix → exits 22 with "smells bad" message
• Interactive selection (no --assignee) prompts sorted member list
• --output json emits structured issue output
• Alias a routes to issue assign
• No assignable members → exits 22 with "smells bad" message
• All 262 tests pass (6 pre-existing LinearCli.GitTest failures unrelated to this change)

Closes: https://linear.app/cryptokairos/issue/CRY-54/add-an-interactive-issue-assign-command

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[bougyman]

Addressed the inline review comment on User.Read.ByTeam: replaced the case with with/when in f6e9e80.

with {:ok, %{"team" => team}} when is_map(team) <- Api.call(document, %{"id" => team_id}),
     %{"members" => %{"nodes" => nodes}} <- team do
  {:ok, Enum.map(nodes, &User.from_map/1)}
else
  {:ok, _} -> {:ok, []}
  error -> error
end

The when is_map(team) guard distinguishes a nil team from a populated one without needing a case.