Expose GOOSE_SESSION_ID environment variable in CLI sessions

[clouatre]

Please explain the motivation behind the feature request.

goose term init sets GOOSE_SESSION_ID for shell integration, but normal CLI sessions (goose, goose run --recipe) do not. This forces recipe authors to generate their own IDs that don't align with Goose's internal session tracking.

Use cases:

• Session-isolated handoff paths (.goose/$GOOSE_SESSION_ID/handoff/)
• Worktree isolation per session (related to Support git worktrees so multiple geese can work independently in git directories #3557)
• Debug correlation between artifacts and session history

Describe the solution you'd like

In crates/goose-cli/src/session/builder.rs, after session creation:

std::env::set_var("GOOSE_SESSION_ID", &session_id);

Subagents inherit parent's ID via environment (for correlation).

Describe alternatives you've considered

• Generate timestamp-based IDs in recipes - works but misaligned with internal IDs
• Query SQLite directly - fragile

Additional context

• Precedent: GOOSE_TERMINAL=1, GOOSE_SUBAGENT_MAX_TURNS already exposed

• Related: Use session IDs as task IDs for subagents instead of UUIDs #4955 (session IDs as task IDs), Support git worktrees so multiple geese can work independently in git directories #3557 (worktree support), feat: Add max_turns to recipe and subagent settings #6198 (recipe settings portability)

• Session ID format (YYYYMMDD_N) contains no sensitive data

• I have verified this does not duplicate an existing feature request

[jhult]

I think this needs to be AGENT_SESSION_ID now (instead of GOOSE_SESSION_ID). See #6626.