Fix missing sentinel file in pipeline execution task prompts

[Iron-Ham]

Summary

• Task instances spawned via the Orchestration 2.0 pipeline path were not writing .claudio-task-complete.json sentinel files because the bridge's BuildTaskPrompt did not include the completion protocol
• The protocol was only injected via --append-system-prompt-file, which silently failed to reach instances
• Embeds the completion protocol directly in the user prompt as the primary mechanism, with the system prompt injection as defense-in-depth

Details

The old execution path (phase/ExecutionOrchestrator) used prompt.TaskBuilder which embedded the completion protocol directly in the user prompt. The 2.0 pipeline path (bridge + bridgewire) used a minimal BuildTaskPrompt that relied entirely on --append-system-prompt-file to inject the protocol via the Claude Code CLI flag. If that flag didn't propagate (version mismatch, path issue, silently ignored), instances had no knowledge of sentinel files.

The bridge package can't import orchestrator/prompt (import cycle), so completionFileName is duplicated as a local constant with documentation pointing to the canonical source.

Test plan

• go test ./internal/bridge/... -race passes
• go test ./internal/orchestrator/bridgewire/... -race passes
• go test ./internal/orchestrator/prompt/... -race passes
• go vet ./... clean
• gofmt -d . clean
• New test assertions verify completion protocol is present when taskID is set and absent when empty