feat: migrate to XDG/FHS install locations

[ukanga]

Closes #21.

Summary

• Binary moves from ~/.wt/wt (shell alias) to ~/.local/bin/wt (on PATH)
• Global config moves from ~/.wt/config.toml to $XDG_CONFIG_HOME/wt/config.toml (default ~/.config/wt/)
• Session state moves from ~/.wt/sessions.json to $XDG_STATE_HOME/wt/sessions.json (default ~/.local/state/wt/)
• macOS gets the same XDG paths, not ~/Library/Application Support/

Changes

src/config.rs / src/session.rs: Hand-rolled xdg_config_home() / xdg_state_home() helpers (avoids dirs::config_dir() / dirs::state_dir() which return Apple paths on macOS). Legacy ~/.wt/ paths are read as a fallback with a one-time stderr notice; writes always go to the new locations.

install.sh: Installs to ~/.local/bin/wt by default. --system flag installs to /usr/local/bin/wt (with sudo). Alias-writing block removed entirely. Re-running the installer migrates legacy files, removes stale shell aliases from rc files, and cleans up ~/.wt/ if empty. Prints a one-line PATH instruction if the install dir is not on $PATH (macOS).

README.md: Updated Installation and Configuration sections.

Test plan

• Fresh install (no ~/.wt/): binary at ~/.local/bin/wt, nothing written under ~/.wt/
• ./install.sh --system installs to /usr/local/bin/wt
• New config/state files land at ~/.config/wt/ and ~/.local/state/wt/
• Existing user re-runs installer: files migrated, ~/.wt/ removed, aliases stripped from rc files, which wt resolves to new path
• Re-running installer after migration is idempotent (exit 0, no duplicates)
• XDG_CONFIG_HOME / XDG_STATE_HOME env vars honoured
• Legacy-only user (no re-install): wt reads from ~/.wt/ with deprecation notice
• cargo test passes (60 unit + integration tests including new XDG and legacy-fallback tests)