Move global config to XDG location (~/.config/projector/config.toml)

[kevdoran]

What & why

Closes #22. Moves the default global config from ~/.projector/projector-config.toml to the XDG-style ~/.config/projector/config.toml, matching the conventions used by gh, mise, and iterm2 and reducing home-directory clutter.

ConfigDir/ConfigFilePath now honor $XDG_CONFIG_HOME:

• If $XDG_CONFIG_HOME is set: $XDG_CONFIG_HOME/projector/config.toml.
• Otherwise: ~/.config/projector/config.toml (XDG default base ~/.config).

Backwards-compatibility / resolution order

config.Load() resolves in this order:

1. New location ($XDG_CONFIG_HOME/projector/config.toml, default ~/.config/projector/config.toml) — preferred.
2. Legacy location (~/.projector/projector-config.toml) — fallback when the new file is absent.
3. Neither present → ErrNotFound (treated as not-yet-configured, same as before).

config.Save() always writes to the new location.

Migration semantics: copy, not move

When the config exists only at the legacy location, Load copies it to the new location (creating ~/.config/projector/), leaves the legacy file untouched, and prints a one-line notice to stdout:

Migrated projector config to <new> (copied from <legacy>; the old file was left in place).

Rationale for copy-vs-move: copying is the safer choice for users who may still run an older pj binary that only reads the legacy path — both binaries keep working. Migration is best-effort: if the copy fails (e.g. read-only ~/.config), Load still loads directly from the legacy file rather than failing the command. Since Save targets the new location, the two files can diverge after a subsequent write; this is the accepted trade-off for keeping older binaries functional, and Load always prefers the new file once it exists.

Tests

Added/updated tests in internal/config/config_test.go:

• new location preferred over legacy (and legacy left unchanged)
• legacy → new copy-migration (new file created, legacy preserved, second load reads new)
• $XDG_CONFIG_HOME override and default-to-~/.config
• Save writes to new location only
• no-config-anywhere → ErrNotFound

The shared test helper now also clears XDG_CONFIG_HOME to keep resolution self-contained.

Docs

Updated docs/commands.md (incl. a note on the XDG location + legacy fallback/migration), CLAUDE.md (config package row), pj-skill.md, and the pj config command Short help text.

Verification

Locally: go build ./cmd/projector, go vet ./..., and go test -race -count=1 ./... all pass.

For reviewers to scrutinize / not verified locally

• Real-world cross-binary behavior (old pj reading legacy while new pj reads/writes the new path) is reasoned about, not exercised in CI.
• The migration message goes to stdout; if any command's stdout is machine-parsed, this one-time line could be unexpected. It only prints on the first load after the file exists solely at the legacy path.
• $XDG_CONFIG_HOME is used verbatim if set; no validation that it is an absolute path (matches typical XDG tooling behavior).

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