Feature Request: Expose Session Name to All Extension Points

Summary

Session naming was added in #2112, but the session name is not accessible during the session. The session name should be available to the model and all extension points (hooks, commands, skills, rules).

Current State

| Context | Session ID | Session Name |
|---------|:----------:|:------------:|
| Model (during conversation) | No | No |
| Hooks (JSON input) | Yes | No |
| Commands ($SESSION_NAME) | No | No |
| Skills | No | No |
| Rules | No | No |
| CLI (resume) | Yes | Yes |

Problem

Users can name sessions, but that name is inaccessible during the session itself. This limits the utility of session naming to just the resume workflow.

Example scenario: A user names a session "vendor-briefing-acme" but cannot:

• Reference the session name in output file names
• Log activity with the session name
• Have commands/skills behave differently based on session type
• Include session context in generated content

Proposed Solution

Expose session name consistently across all extension points:

1. Model Context

Include session info in the system context provided to the model:

Session name: vendor-briefing-acme
Session ID: 550e8400-e29b-41d4-a716-446655440000

2. Hooks (JSON Input)

Add session_name alongside existing session_id:

{
  "session_id": "550e8400-e29b-41d4-a716-446655440000",
  "session_name": "vendor-briefing-acme",
  "transcript_path": "...",
  "hook_event_name": "PreToolUse"
}

3. Commands (Variable Substitution)

Support $SESSION_NAME and $SESSION_ID variables in command files:

# Activity Log

Logging activity for session: $SESSION_NAME

Output file: logs/$SESSION_NAME-activity.md

4. Skills

Same variable substitution in SKILL.md files, or access via the model's system context.

5. Rules

Session context available when rules reference session-specific behavior.

Use Cases

1. Activity logging: Hooks log [session-name] Tool: Read file.md instead of [abc123] Tool: Read file.md

1. Output organization: Commands generate files like vendor-briefing-acme-prep.md based on session name

1. Session-aware workflows: A "research" session could auto-activate research-focused skills

1. Traceability: Generated content can reference which session produced it

1. Multi-session coordination: Skills could check if related sessions exist

Alternatives Considered

1. Hooks-only: Partial solution; doesn't help commands, skills, or model context
2. Environment variable: Would work for hooks/bash but not for model or variable substitution
3. Status quo: Session names remain decoration-only, limiting their utility

Additional Context

This builds on #2112 (session naming). The implementation likely requires:

• Adding session_name to hook input schema
• Adding session context to model system prompt
• Supporting variable substitution for $SESSION_NAME in commands/skills

Backward compatible: unnamed sessions could expose session_name: null or use the session ID as fallback.