Fix nil pointer crash in bd export command

When running `bd export -o .beads/issues.jsonl`, the command crashes with a nil pointer dereference.

## Error
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x108 pc=0x1034456fc]

goroutine 1 [running]:
main.init.func14(0x103c24380, {0x1034a9695?, 0x4?, 0x1034a95c9?})
        /Users/stevey/src/vc/adar/beads/cmd/bd/export.go:74 +0x15c
```

## Context
- This happened after closing bd-86, bd-91, bd-92
- Auto-export from daemon still works fine
- Only the manual `bd export` command crashes
- Data was already synced via auto-export, so no data loss

## Location
File: `cmd/bd/export.go` line 74


Add --global flag to daemon for multi-repo support

Currently daemon creates socket at .beads/bd.sock in each repo. For multi-repo support, add --global flag to create socket in ~/.beads/bd.sock that can serve requests from any repository.

Implementation:
- Add --global flag to daemon command
- When --global is set, use ~/.beads/bd.sock instead of ./.beads/bd.sock
- Don't require being in a git repo when --global is used
- Update daemon discovery logic to check ~/.beads/bd.sock as fallback
- Document that global daemon can serve multiple repos simultaneously

Benefits:
- Single daemon serves all repos on the system
- No need to start daemon per-repo
- Better resource usage
- Enables system-wide task tracking

Context: Per-request context routing (bd-92) already implemented - daemon can handle multiple repos. This issue is about making the UX better.

Related: bd-2 (parent issue for multi-repo support)


Document multi-repo workflow with daemon

The daemon already supports multi-repo via per-request context routing (bd-92), but this isn't documented. Users need to know how to use beads across multiple projects.

Add documentation for:
1. How daemon serves multiple repos simultaneously
2. Starting daemon in one repo, using from others
3. MCP server multi-repo configuration
4. Example: tracking work across a dozen projects
5. Comparison to workspace/global instance approaches

Documentation locations:
- README.md (Multi-repo section)
- AGENTS.md (MCP multi-repo config)
- integrations/beads-mcp/README.md (working_dir parameter)

Include:
- Architecture diagram showing one daemon, many repos
- Example MCP config with BEADS_WORKING_DIR
- CLI workflow example
- Reference to test_multi_repo.py as proof of concept

Context: Feature already works (proven by test_multi_repo.py), just needs user-facing docs.