Expose /rename'd session name in model prompt context

Problem

The session name set via /rename <name> is used for local UI/history only.
It is not surfaced to the model's prompt context — the model has no way to read its own session title.

Why this would help

Users routinely rename sessions to a ticket ID or short topic
(e.g. ABC-1293, Refactor auth). If the model could read this:

• Commit message suggestions / PR titles / branch names could automatically reflect the session topic — no need to repeat the ticket ID in chat each time.
• The model has an explicit anchor to the user's stated goal across long conversations, reducing drift ("what were we doing again?").
• Aligns naturally with auto memory and CLAUDE.md as a per-session context layer.

Suggested mechanisms (non-prescriptive)

• Add a field to the env block (alongside cwd, Platform, etc.).
• Or expose via env var readable from hooks (so users can opt-in via additionalContext).

Related (distinct) issues

• #47679 — programmatic call of /rename from tools/skills (different direction)
• #44281 / #55397 — session name in terminal title (different surface)
• #48868 — claude.ai → local CLI rename propagation (different problem)

This request is specifically: the model reading its own session title in context.