fix(daemon): clear error for over-length socket path

[olddognewflex]

Closes #68.

Problem

When qid's resolved socket path exceeds the OS sockaddr_un.sun_path limit (104 bytes on darwin, 108 on Linux), net.Listen("unix", …) fails with a bare bind: invalid argument. qid exits immediately and qi daemon start reports only "did not become ready", with the real reason buried in qid.log — the failure looks like a permissions or path bug, not a length bug.

Fix

daemon.Listen now length-checks the path before net.Listen and returns a clear error naming the limit and the remedy:

socket path is N bytes, over the M-byte OS limit for unix sockets on this platform; use a shorter --socket or XDG_RUNTIME_DIR

The limit is platform-specific, so it lives behind a socket_darwin.go / socket_other.go build-tag split (maxSocketPathLen = 103 usable on darwin, 107 elsewhere — sun_path size minus the NUL), matching the existing dataless_darwin.go / dataless_other.go precedent.

Purely additive: a clearer error in front of an already-fatal condition. Per the issue, actually working around the limit (chdir + relative name, hashing) is out of scope.

Tests

• Listen rejects an over-length path with a message naming the limit and remedy, before any filesystem side effect.
• A normal short path still binds (regression guard).

Full go test ./..., go vet, go build ./... green.