feat(tui): expose session header slot

[kitlangton]

What

Adds a V2 TUI plugin slot at the top of the active session view. Plugins can use session.header for persistent session-level context such as environment, branch, workflow mode, or warnings.

context.ui.slot("session.header", ({ sessionID }) => (
  <text>Review mode · {String(sessionID)}</text>
))

How

• Mounts PluginSlot before the session transcript region.
• Passes the active sessionID to every registered contribution.
• Adds a CLI patch changeset.

Scope

This PR only exposes the generic host position. It does not add a built-in contribution or change plugin loading.

Testing

• bun typecheck in packages/tui.
• bun run test in packages/tui: 265 passed, 1 skipped.
• Pre-push bun turbo typecheck --concurrency=3: 32 package tasks passed.
• git diff --check passed.

Demo

A temporary local plugin renders session-level review state through session.header. The demo contribution is not included in this PR.