feat(prompt): make the plan handoff a versioned, validated contract

helexa#179's application-owned system prompts let tireless shape both ends of
the cc->oc handoff, so the "will a 27B execute an Opus plan" risk becomes a
tunable rather than a hope.

Three prompts, versioned as one set: plan.cc.md tells Claude Code it is writing
for a literal, absent reader; implement.oc.md tells OpenCode to execute exactly
that and report rather than improvise; implement.cc.md covers unplanned issues.
PromptSet::load refuses a mismatched set, and tests assert the prompts mention
every section ChildSpec requires.

The middle is validated, not trusted. plan::validate rejects a plan before any
implementation job is enqueued unless every child carries a runnable acceptance
command (a stopping condition) and a non-empty out-of-scope list (a boundary) --
the two sections a small model needs and a human reader does not. Dangling and
cyclic dependencies are caught too, and implementation_order derives the start
order.

cc uses --append-system-prompt, never --system-prompt: replacing Claude Code's
default discards the tool-use scaffolding that makes it a coding agent. Pin
bumped to 2.1.220, the version this flag surface was verified against.

The oc path depends on helexa#179's passthrough guarantee, which is still open
and unverified for qwen3 arch templating. Stage 5 now opens with a PONG probe
rather than debugging it through a failed implementation run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DHhHtohxcdk1PL3tfnYJdH

script/infra-setup.sh

@@ -106,7 +106,7 @@ Remaining one-time steps (operator, on the target host):
    The OAuth flow is interactive and must be completed *as the service account*,
    because Claude Code reads credentials from $HOME:
 
-       sudo -u tireless -H /usr/bin/npx -y @anthropic-ai/claude-code@2.1.119
+       sudo -u tireless -H /usr/bin/npx -y @anthropic-ai/claude-code@2.1.220
        # then: /login, and complete the browser flow
 
    This writes /var/lib/tireless/.claude.json. The token refreshes in place,