Add non-destructive context export for cross-session knowledge sharing

Problem

When working on long sessions, there's no way to share context with a new session without compacting the current one. /compact is destructive — it replaces the full context with a summary in-place.

Common scenario: I'm deep in a debugging session and want to spin up a parallel session for a related task. The new session has zero context about what I've been doing.

Suggested feature

A non-destructive context export/import mechanism:

# In current session (context stays intact):
/export-context [file-or-label]

# In new session (loads the summary as initial context):
/import-context [file-or-label]

How it would work

• /export-context generates a structured summary of the current session (decisions made, files changed, key findings, current state) and saves it — without modifying the current session's context
• /import-context loads that summary into a fresh session, giving the new agent immediate awareness of prior work
• This is fundamentally different from /compact (destructive, in-place) and /resume (continues same session, no parallelism)

Current workarounds

• MEMORY.md: Persistent but limited (200 lines), requires manual curation, and is global (not session-specific)
• Manual file export: Ask Claude to "write a summary to a file", then in new session "read that file" — works but clunky
• /resume: Continues the same session rather than creating a new one with shared context

Use cases

1. Parallel work streams: Debug session + implementation session on related code
2. Session handoff: End of day summary → next morning pickup in fresh session
3. Team collaboration: Export context for another developer's Claude session
4. Long projects: Periodic context snapshots without losing full conversation detail

Bonus ideas

• Auto-structured export format (files touched, decisions, TODOs, current blockers)
• Named context snapshots (multiple exports from same session)
• Expiring/timestamped contexts for relevance tracking