## Summary

Closes wesm#86

Windows users writing native backslash paths in `config.toml` (e.g.
`data_dir = "C:\Users\wesmc\msgvault"`) get cryptic TOML parse errors
because backslashes are escape characters in TOML double-quoted strings.
This PR addresses the problem from two angles:

**1. `--home` CLI flag** — Sets the home directory directly from the
command line, equivalent to `MSGVAULT_HOME`. Config, database, tokens,
attachments, and analytics all live under this directory. If a
`config.toml` exists there, it's loaded.

```
msgvault.exe --home "C:\Users\wesmc\msgvault" sync-full [email protected]
```

**2. Improved TOML error messages** — When `config.toml` contains
backslash paths that fail to parse, the error now includes a hint:

```
hint: Windows paths in TOML must use forward slashes (C:/Games/msgvault)
or single quotes ('C:\Games\msgvault').
```

This catches both `\G`-style "invalid escape" errors and `\U`-style
"expected hexadecimal digits" errors.

## Changes

- `cmd/msgvault/cmd/root.go` — Add `--home` persistent flag, pass it
into `config.Load()`
- `internal/config/config.go` — `Load()` accepts a `homeDir` parameter
that overrides `DefaultHome()` before determining config path; detect
backslash-related TOML errors and wrap with hint
- `internal/config/config_test.go` — Tests for `--home` (sets
HomeDir/DataDir, reads config.toml from home dir, expands tilde), tests
for backslash hints (`\G` and `\U` variants)
- `CLAUDE.md` — Add General Workflow, Go Development, and Git Workflow
guidelines

## Test plan

- [x] `make test` passes
- [x] `make lint` passes
- [x] Manual: `msgvault.exe --home "C:\Users\wesmc\testing" init-db`
uses the override directory
- [x] Manual: config.toml with `data_dir = "C:\Games\msgvault"` shows
the hint
- [x] Manual: config.toml with `data_dir = "C:\Users\wesmc\data"` shows
the hint (`\U` variant)
- [x] Manual: config.toml in `--home` directory is loaded (e.g.
`rate_limit_qps` applies)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>