Expose Lody tasks, subagents and usage over ACP

[leynos]

Summary

This branch makes VTCode's ACP server negotiate and publish the Lody task,
subagent, and provider-usage surfaces that it actually implements. It replaces
the ignored private lifecycle notification with standard ACP tool updates,
adds session-scoped management for owned foreground and background work, and
emits normalized per-response usage without inventing absent provider data.

It also adds an explicit, proxy-safe supports_stream_usage opt-in for custom
OpenAI-chat providers. This allows compatible Baseten profiles to request and
retain terminal usage chunks while leaving existing custom-provider request
bodies unchanged.

ExecPlan: ACP Lody task lifecycle negotiation

Review walkthrough

• Start with the Lody adapter for standard task updates, capability negotiation, ownership filtering, and the three management operations.
• Review the usage adapter for the canonical _lody/session/usage_update mapping and absent-usage behaviour.
• Follow the ACP handler integration and duplex tests to see initialization, per-response publication, task-loop deltas, and session-scoped management over the official ACP channel.
• Inspect background task ownership and its regression coverage for fail-closed legacy loading, bounded output tails, and cross-session isolation.
• Finish with the custom-provider profile field, wiremock playback, and vidai-mock physics test.

Validation

• Focused ACP capability, mapping, property, recovery, and duplex tests: 9 passed.
• Custom-provider configuration tests: 4 passed.
• Wiremock terminal-usage playback: 1 passed.
• Vidai-mock Baseten-shaped streaming physics: 1 passed.
• ./scripts/check.sh: 6,588 tests passed, 16 skipped; 33 harness tests passed, with formatting, security, logging, governance, Clippy, build, PTY/TUI, and documentation checks green.
• cargo install --path . --locked --force: installed successfully at user level after each code/documentation milestone.

Notes

• Lody usage capability version 1 is always advertised; a notification is sent only when a successful provider response contains normalized usage.
• Subagent management and background-task capability flags are conditional on the corresponding VTCode controller/runtime support.
• Legacy ownerless background records remain loadable but are hidden from ACP management; VTCode never assumes ownership after restart.
• Scheduled tasks, richer provider-specific usage detail, provider limits, compaction notices, and final-answer phase metadata remain independent follow-ups.
• The configuration reference generator completed successfully, but its unrelated pre-existing full-file drift was excluded; this branch retains only the two new schema rows.

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