Make `--name` create or resume a session

[MattPD]

Describe the feature or problem you'd like to solve

copilot --name <name> creates a new named session, but it does not resume an existing session with that name. After the first launch, users and automation must know whether the session exists before selecting --name <name> or --resume <name>.

The same distinction breaks CLI relaunches. A session started with copilot --name project-a can fail during /update when the replacement process receives both the original --name and an internal --session-id:

error: option '-n, --name <name>' cannot be used with option '--session-id <id>' when it resolves to an existing or remote session or task.
--name only names a brand-new session created by --session-id=<new-uuid>; the resolved session was not renamed.

Proposed solution

Give copilot --name <name> create-or-resume behavior. --name should use the same name lookup and matching rules as --resume <name>:

1. If no session matches, create a session with that name.
2. If exactly one session matches, resume that session.
3. If multiple sessions match, follow the current --resume <name> ambiguity behavior. Do not create another session.
4. Resume the selected session when /restart, /update, or another command relaunches the CLI. A retained --name should not fail because the session now exists or because the relaunch also carries an internal session ID.

--resume should remain available for three uses: session IDs, the session picker, and existing workflows. Users and automation could then use one repeatable command whenever a stable name identifies the session.

Example prompts or workflows

1. The first copilot --name project-a invocation creates a named session.
2. A later copilot --name project-a invocation resumes that session.
3. /update or /restart resumes that session without a flag conflict.
4. An alias or script can keep invoking copilot --name project-a without first querying session state.

Additional context

Issue #3332 reports /restart failing in a session created with --name. The requested semantics address that failure and the broader need for one repeatable name-based command.