feat(maestro): add maestro-install wizard (cargo install + systemd)

New `maestro-installer` crate exposing the `maestro-install` binary. Bootstrap
flow: `cargo install --git ... maestro-installer && maestro-install` runs
`cargo install` for `maestro-daemon` and `maestro-cli`, optionally drives
`maestrod configure`, and on systemd hosts writes a user unit at
`~/.config/systemd/user/maestrod.service` plus reload/enable/start. Pure
builders (cargo install args, systemctl args, unit body) live in the lib and
are unit-tested; main.rs is the prompt glue.

Refs NAP-225.

CLAUDE.md

@@ -22,10 +22,13 @@ crates/
 ├── maestro-daemon/      # per-host tmux wrapper + WS endpoints (NAP-212, NAP-214)
 ├── maestro-registry/    # always-on phone book + HTTP API (NAP-213, NAP-215)
 ├── maestro-cli/         # `maestro list/spawn/attach/kill` (NAP-216)
+├── maestro-installer/   # `maestro-install` wizard: cargo install + systemd (NAP-225)
 └── maestro-e2e/         # cross-process integration tests (NAP-217)
 web/                     # browser client (NAP-218, plan 2)
 ```
 
+See `docs/INSTALL.md` for end-user install instructions.
+
 ## Conventions
 
 - **TDD by default.** Failing test first, then minimal impl. Commit test + impl together.