Architecture: Unify MCP and REPL code paths

[evansenter]

Summary

There's significant duplication between MCP server mode and REPL mode. This makes maintenance harder and leads to inconsistencies (e.g., #34 cancellation was MCP-only).

Current Duplication

Concern	MCP	REPL
Session management	McpSession HashMap	interaction_id variable
Output	JSON response + file log	termimad to terminal
Cancellation	Task abort (#34)	CANCELLED AtomicBool
Tool execution	Same run_interaction()	Same

Proposed Unification

1. Output Sink Trait

trait OutputSink: Send + Sync {
    fn emit_text(&self, text: &str);
    fn emit_tool_call(&self, name: &str, args: &Value, duration: Duration);
    fn emit_error(&self, error: &str);
}

struct TerminalSink { skin: MadSkin }
struct McpSink { tx: Sender<String> }
struct FileSink { file: File }

2. Unified Session Manager

struct SessionManager {
    sessions: HashMap<String, Session>,
}

struct Session {
    id: String,
    interaction_id: Option<String>,
    cancellation: CancellationToken,
}

3. Shared Cancellation

Use tokio_util::sync::CancellationToken instead of AtomicBool. Can be cloned and checked at any await point.

Scope

This is a significant refactor. Consider:

• Incremental migration (one concern at a time)
• Keep run_interaction() as the shared core
• Add abstractions only where duplication causes bugs

Related

• Move off auto_functions for finer-grained ctrl-c cancellation #31 - Ctrl+C handling (blocked on this?)
• MCP: Handle notifications/cancelled to abort running requests #34 - MCP cancellation (partial fix)

[evansenter]

Progress update (020bcd2):

Completed:

• ✅ CancellationToken replaces global AtomicBool
• ✅ Per-interaction tokens (cleaner than global state)
• ✅ Removed redundant eprintln calls from bash.rs (prep for OutputSink)

Remaining:

• OutputSink trait for unified output routing (terminal vs file vs MCP JSON)
• Session manager unification

[evansenter]

Progress Update

Completed via incremental approach:

1. Shared Cancellation ✅

Commit 020bcd2 - Replaced global AtomicBool with tokio_util::sync::CancellationToken:

• Per-interaction cancellation tokens
• MCP threads cancellation token through request handling
• REPL creates token per interaction

2. Output Sink Trait ✅

Commit 2b5950b - Added OutputSink trait for unified output routing:

• FileSink for MCP mode (log files only)
• TerminalSink for REPL mode (stderr + log files)
• log_event/log_event_raw route through configured sink
• Tool output now appears in REPL terminal

3. Unified Session Manager - Deferred

The current session management approach works well:

• MCP uses interaction_id passed to Gemini
• REPL stores interaction_id locally

No bugs from current duplication, so deferring SessionManager until needed.

The core goals of this issue (consistent cancellation, unified output) are now addressed.