feat: expose AGENT_SESSION_ID env var to extension child processes

[tlongwell-block]

Inject the agent session ID as an environment variable into extension child processes and shell commands, enabling scripts and tools to identify which goose session they belong to.

Two injection paths cover all extension types that spawn processes:

• Stdio extensions: session_id is inserted into the merged env HashMap before spawning the child process via Command::envs()
• Developer shell (builtin): session_id is extracted from MCP request metadata headers per command and pushed into ShellConfig.envs

Per-child-process injection (not process-wide std::env::set_var) keeps this safe for concurrent sessions in goose-server.

Closes #6308

[tlongwell-block]

/goose

[github-actions]

Summary: This PR adds AGENT_SESSION_ID environment variable injection to extension child processes via two paths: stdio extensions (via ExtensionManager's env HashMap) and developer shell commands (via MCP metadata headers). The implementation follows existing patterns and includes good security practices.

✅ Highlights

1. Good security practice: Null-byte filtering was added for both working_dir and session_id in rmcp_developer.rs:

   fn extract_session_id_from_meta(meta: &MetaExtensions) -> Option<String> {
       // ...
       .filter(|s| !s.is_empty() && !s.contains('\0'))

   This prevents potential command injection via null bytes in the metadata.

2. Consistent pattern: The implementation follows the existing WORKING_DIR_HEADER pattern for the new SESSION_ID_HEADER, making the code predictable and maintainable.

3. Two-path approach is correct: Stdio extensions receive the env var at process spawn time via ExtensionManager, while the developer extension receives it per-command via MCP metadata. This correctly handles the different lifecycles.

4. Environment variable name consistency: AGENT_SESSION_ID is used consistently across CLI (term.rs), extension_manager.rs, and rmcp_developer.rs.

🟢 Suggestions

1. Constant duplication (crates/goose-mcp/src/developer/rmcp_developer.rs:18-19): SESSION_ID_HEADER and WORKING_DIR_HEADER are duplicated between session_context.rs and rmcp_developer.rs.

   This is acceptable since goose-mcp can't depend on goose (would be circular or undesirable), and follows the pre-existing pattern. Consider adding a comment noting the intentional duplication:

   // Note: Intentionally duplicated from goose/src/session_context.rs 
   // since goose-mcp cannot depend on the goose crate
   const SESSION_ID_HEADER: &str = "agent-session-id";

2. Consider documenting the env var: Since this is a user-facing feature (scripts can now access AGENT_SESSION_ID), consider adding documentation about:

   • When/where the env var is available
   • The format of session IDs (e.g., YYYYMMDD_N)
   • Any caveats (e.g., not available for frontend tools)

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Review generated by goose

[codefromthecrypt]

looks good to me!

I might as a follow-up see if I can get a test for this maybe a fake command that echo's the ENV. Don't block on this.