feat: /models, /agents, and /cancel slash commands with ACP configOptions

[chaodu-agent]

Summary

Implements /models, /agents, and /cancel Discord slash commands that bridge to ACP session config options and cancellation.

Closes #477

Slash Commands

Command	Action
/models	Select AI model from a dropdown (ephemeral)
/agents	Select agent mode from a dropdown (ephemeral)
/cancel	Cancel the current in-flight operation

How it works

/models and /agents

User: /models (or /agents)
  → OpenAB reads cached configOptions from ACP session
  → Renders Discord Select Menu with available options
  → User picks an option
  → OpenAB sends session/set_config_option (or falls back to prompt)
  → Discord message updated with "✅ Switched to X"

/cancel

User: /cancel (while bot is streaming)
  → OpenAB sends session/cancel notification to ACP agent
  → Agent aborts in-flight LLM requests and tool calls
  → User sees "🛑 Cancel signal sent."

Changes

File	Change
src/acp/protocol.rs	Add ConfigOption types + parse_config_options() with kiro-cli fallback
src/acp/connection.rs	Cache configOptions, set_config_option() with prompt fallback, send_notification(), cancel_handle()
src/acp/pool.rs	get_config_options(), set_config_option(), cancel_session() with lock-free cancel handles
src/adapter.rs	pool() accessor on AdapterRouter
src/discord.rs	Register /models + /agents + /cancel, interaction handlers

Backend compatibility

Backend	configOptions	set_config_option	session/cancel
Codex (codex-acp)	✅ Standard	✅ Native	✅ Native
Kiro (kiro-cli)	⚠️ models/modes fallback	⚠️ Prompt fallback	✅ Native

Key design decisions

• Lock-free cancel: Cancel handles (stdin + session_id) stored separately from the connection mutex to avoid deadlock during streaming
• Thread key format: Uses "discord:<channel_id>" matching the adapter router format
• Ephemeral responses: Select menus and cancel confirmations only visible to invoking user

Tested

• ✅ cargo check — no errors
• ✅ /models shows 12 models from kiro-cli
• ✅ /agents shows 2 modes (kiro_default, kiro_planner)
• ✅ Model/agent selection works with prompt fallback
• ✅ /cancel stops in-flight streaming without deadlock
• ✅ "No options available" when no session exists

---

Discord Discussion URL: https://discord.com/channels/1486155598964719616/1495343752947040306

[thepagent]

The CI failure is from cargo clippy -- -D warnings on the merge commit:

warning: methods `send_notification` and `cancel_session` are never used

AcpConnection::cancel_session() and send_notification() are defined but never called — the pool's cancel_session() uses the lock-free stdin approach directly, bypassing the connection-level methods.

This only surfaces when merged with main (the branch itself compiles clean). Fix options:

1. Rebase onto main and add #[allow(dead_code)] on send_notification and cancel_session in connection.rs (keeps them as public API for future use)
2. Or remove them — the pool's lock-free cancel is the only path used

The branch needs a rebase regardless (needs-rebase label) since it's based on v0.6.4.

[thepagent]

CI fix: rebase required

The cargo clippy -- -D warnings failure is:

warning: methods `send_notification` and `cancel_session` are never used
  --> src/acp/connection.rs:293:18

Root cause: The branch is based on v0.6.4 — the GitHub merge commit creates these methods but nothing calls them because pool.cancel_session() uses the lock-free stdin approach directly.

Attempted rebase: connection.rs conflicts are trivial (both-sides field additions), but discord.rs has diverged too much between v0.6.4 and current main (different imports, struct patterns, handler signatures). A mechanical rebase won't work.

Fix needed: Rewrite the PR against current main (openab-0.7.8-beta.7). The core logic (configOptions parsing, slash command handlers, cancel handles) is solid — it just needs to be applied to the current codebase structure.

Alternatively, add #[allow(dead_code)] on send_notification and cancel_session in connection.rs if you want to keep them as public API for future use.

[thepagent]

LGTM — clean rebase, much simpler than the previous version.

✅ configOptions parsing + caching on create/load
✅ ConfigUpdate event keeps cache fresh during streaming
✅ Pool lock ordering correct (drop state before locking connection)
✅ Ephemeral responses, select menu with current value pre-selected
✅ No dead code — previous send_notification/cancel_session removed

Notes:

• set_config_option will error for backends that don't support it (kiro-cli). Previous version had a prompt fallback — acceptable to omit if codex-acp is the primary target, but worth documenting.
• docs/slash-commands.md references /models, /agents, /cancel but this revision only implements /model. I'll push a fix to align the docs.