fix(QUA-576): route -p and positional args through configured cliAgent

[Desperado]

Summary

Fixes QUA-576.

Before this change, both non-interactive entry paths in main.go called ag.Run (the Anthropic API agent) unconditionally — silently ignoring backend=cc (and backend=codex). Every CI / scripting / cloud-session invocation that used qmax-code -p "..." or qmax-code "..." was bypassing the cc backend and hitting the Anthropic API directly.

Repro (pre-fix)

$ qmax-code config           # shows backend = "cc" (claude found: …)
$ qmax-code -backend cc -p "hi"
Error: API call failed: API error 401: invalid x-api-key

The 401 comes from a stale keychain key being sent to Anthropic — not from any actual CC backend invocation.

Repro (post-fix)

$ qmax-code -backend cc -p "Reply with exactly one word: alpha"
alpha

Fix

• Single runOneShot dispatcher shared by both -p and positional-arg modes.
• If cliAgent is set (cc/codex), the prompt goes through it (cliAgent.Run) with its own streaming output via a Terminal created on demand.
• If cliAgent is nil, fall back to ag.Run exactly as before.

This also restores doc-comment accuracy: README/CONTRIBUTING/cc_agent.go all claim "no Anthropic API key needed when backend=cc" — that statement is now true for non-interactive mode, not just the REPL.

Tests

• main_oneshot_test.go (new) — fakeCLIAgent + 3 assertions:
  • CLIAgent is preferred when set
  • API path is used as fallback when CLIAgent is nil
  • Errors from the CLIAgent propagate (so main.go exits non-zero)
• Full suite green: go test ./... -short -count=1.

Test plan

• qmax-code -backend cc -p "..." returns through claude, no Anthropic 401.
• qmax-code -backend cc "..." (positional) same behaviour.
• qmax-code -backend api -p "..." still uses the API path (or errors cleanly if no key).
• qmax-code -backend codex -p "..." routes through codex CLI.
• CI integration (relevant to QUA-404): cloud sessions driving qmax-code -p now actually use the configured backend.

🤖 Generated with Claude Code

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[Desperado]

Included and merged via #103 (db91e5d). Closing this stale duplicate PR to avoid re-merging the same change.