stop hard-coding "root" as the default agent name

[dgageot]

Removes the remaining places that hard-coded "root" as the default/initial agent name and routes every "default agent" lookup through a single helper.

What changed

• New team.Team.AgentOrDefault(name) method: returns the agent identified by name, falling back to team.DefaultAgent() when name is empty. Centralizes the "empty means default" pattern that was being open-coded in many places.
• All CLI commands with an --agent flag now default to "" (the empty string) instead of "root". The flag is resolved against the team via AgentOrDefault, so any agent name works.
• Every hard-coded t.Agent("root") / WithCurrentAgent("root") call site replaced with a resolved name (AgentOrDefault / DefaultAgent) — cmd/root/run.go, pkg/a2a/adapter.go, pkg/acp/agent.go, pkg/server/server.go, pkg/server/session_manager.go.
• pkg/runtime/remote_runtime.go: currentAgent now defaults to "" (server-side resolves), RunStream no longer special-cases the literal "root", and a sync.Once-cached resolvedAgent(ctx) helper keeps CurrentAgentName() cheap on hot paths.
• cmd/root/completion.go: shell completion now mirrors team.DefaultAgent() (prefer "root" if present, otherwise the first agent declared) so completion stays in sync with what the runtime would actually run.
• Comments and the LLM-creator instructions wording updated to match the new reality.
• Added unit tests for Team.DefaultAgent and Team.AgentOrDefault covering the empty-team / has-root / no-root / explicit-name / unknown-name paths.

Backwards compatibility

team.DefaultAgent() keeps preferring an agent literally named "root" when one exists, so any existing config / API consumer that relied on that name continues to behave identically. Configs without a root agent now resolve to the first declared agent instead of failing.