Feature Request: Cross-environment session resume (local ↔ Codespace)

## Problem

There's no supported way to resume a Copilot CLI session across environments. A common workflow is starting work locally, then needing to continue in a Codespace (or vice versa). Today, `--resume` only works for sessions created in the same environment.

## Current behavior

I manually copied a session directory (`~/.copilot/session-state//`) from my local machine to a Codespace and ran `copilot --resume `. The CLI found the session directory but **overwrote** `events.jsonl` with a fresh session start event, destroying the transferred conversation history. The session had no prior context.

## Desired behavior

A first-class way to continue a session in a different environment. This could take many forms — some options:

1. **Make `--resume` work with transferred session directories** — restore conversation history from the existing `events.jsonl` rather than overwriting it
2. **Cloud-backed sessions** — sessions stored server-side so `--resume ` works from any authenticated environment
3. **Import from `/share` output** — `/share gist` already exports sessions; a `/import ` or `copilot --resume-from ` could restore context from that export

## Security considerations

Cross-environment session transfer raises real security questions that should inform the design:

- **Session data is sensitive.** `events.jsonl` contains the full conversation history — source code, command output, file contents, and potentially secrets or credentials that appeared in the session. Any transfer mechanism needs to treat this data carefully.
- **Authentication boundary.** Sessions reference tools, MCP servers, and permissions that may differ between environments. A resumed session shouldn't inherit elevated permissions from the source environment.
- **Shared environments.** If sessions can be transferred to Codespaces, there's a risk of session data landing in shared or org-wide Codespaces where others have access. The mechanism should warn or prevent transfer to untrusted environments.
- **Gist-based transfer.** If `/share gist` output becomes importable, secret gists are still accessible to anyone with the URL. Importing from gists should be opt-in and warn about the sensitivity of the data.
- **Replay safety.** Replaying `events.jsonl` in a new environment could re-execute tool calls (bash commands, file writes) that made sense in the original environment but are dangerous in the new one. Resume should restore *context*, not *replay actions*.

## Workaround

Today I use `/share gist` to export the session, then manually paste a summarized handoff prompt into a new session in the target environment. This works but loses the structured session state (plan, todos, session DB, file tracking).

## Environment

- Copilot CLI v0.0.415
- macOS → GitHub Codespace transfer