[FEATURE] Let --resume continue a Claude app conversation in Claude Code

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

Conversations in the Claude app (mobile/web) and Claude Code sessions are stored separately. `claude --resume` only sees sessions in `~/.claude/projects` for the current directory, so there is no way to carry a conversation started in the Claude app into Claude Code.

This is a common pattern: think through a problem on a phone, then move to a machine to actually do the work. Today the only workaround is to ask Claude to write a summary block and paste it in manually as the first message, which loses detail and takes an extra round trip.

### Proposed Solution

- Let `--resume` accept a Claude app conversation, e.g. `claude --resume --from-app` opening a picker of recent claude.ai conversations, or `claude --resume `
- Import the conversation history as context for a new local session

### Alternative Solutions

- A "Continue in Claude Code" action on a conversation in the Claude app that generates a resumable session ID
- An export/import command pair

### Priority

Medium - Would be very helpful

### Feature Category

CLI commands and flags

### Use Case Example

I think through a design problem in the Claude app on my phone (planning, tradeoffs, a school/project decision), then sit down at my machine to do the actual work. Right now that context does not follow me: I have to ask Claude to emit a summary block and paste it into a fresh Claude Code session as message one, losing the detail of the original conversation.

### Additional Context

Related, but distinct:

- #35555 (resume in the browser interface)
- #50067 (resume in the desktop app)

Both ask for resume parity across Claude Code surfaces. This is a different ask: crossing from the Claude app into Claude Code.

Start by finding where `claude --resume` is implemented and how sessions in `~/.claude/projects` are discovered. Then determine whether Claude app conversations are available to Claude Code at all. Done would mean a supported flag or argument can import a selected app conversation into a new local session, with CLI behavior covered by tests or docs.

Written by the indexing model from the issue text.