fix(cli): make ao spawn --name required again

[harshitsinghbhandari]

Summary

Reverts only the --name relaxation from #2411, keeping that PR's agent catalog (ao agent ls) and spawn auth preflight fully intact.

#2411 made ao spawn --name optional with a prompt-derived fallback (deriveDisplayNameFromPrompt). The derived names are low quality (first prompt words truncated at 20 runes, e.g. "Work on thread PR") and defeat the point of the original required flag from #2302: deliberate, readable sidebar names. Names are load-bearing for dashboard UX.

What changed (CLI only)

• --name is required again on ao spawn, rejected client-side when missing (and the existing >20-rune cap is kept), before any daemon call.
• Removed resolveSpawnDisplayName / deriveDisplayNameFromPrompt.
• Restored the --name help text and the non-omitempty displayName request tag.
• Tests: added coverage that spawn without --name fails; the resolution/preflight tests that previously relied on the derived path now pass an explicit --name.

Net diff is negative: spawn.go +7 / -33.

Explicitly NOT touched

• Agent catalog (ao agent ls, --refresh, --json) and the spawn auth preflight / --skip-agent-check from feat(cli): context-aware agent spawn with agent catalog and auth preflight #2411 stay as-is.
• Context-aware project/agent resolution from feat(cli): context-aware agent spawn with agent catalog and auth preflight #2411 is preserved.
• Daemon stays as is: displayName remains optional at POST /sessions with the 20-char cap. The desktop new-task dialog depends on omitting it, so the API contract is unchanged.

Tests

cd backend && go test ./internal/cli passes. Full go build ./... passes.

Closes #2849

🤖 Generated with Claude Code

[nikhilachale]

ao spawn --name — that just got made optional, so we can directly ask the orchestrator using this CLI command without requiring --name, since the orchestrator has the context of the project name. For the CLI, we do have to pass the project name. If we write about it in the docs, I think we don't need to make the name required.

[harshitsinghbhandari]

Thanks @nikhilachale. To clarify the intent: --name is required for the dashboard UX, not for the orchestrator. The whole purpose (from #2302) is that a user glancing at the sidebar can tell what each worker agent is working on. A deliberate, readable label is what makes that work.

The optional path doesn't serve that: it doesn't use the project name, it runs deriveDisplayNameFromPrompt, which is the prompt's first words truncated at 20 runes (e.g. "Work on thread PR"). That truncated-prompt string is exactly the low-quality sidebar label #2849 is about, so documenting the optional behavior wouldn't fix the readability problem it creates.

The orchestrator/API convenience case is untouched here: the daemon still keeps displayName optional at POST /sessions (with the 20-char cap), so anything spawning over the API can still omit it. This PR only tightens the human-facing CLI, where a chosen name is the point.

[nikhilachale]

LGTM