add tier-2 CLI shortcuts and clide tail --events

`bin/clide` gains the single-word shortcuts CLAUDE.md's tier 2 spells
out: open, active, insert, replace-selection, save. Each maps the
flat positional argv into the canonical editor.* IPC shape. Insert
and replace-selection accept a lone `-` to read text from stdin so
piping works (`pbpaste | clide replace-selection -`).

`clide tail --events` is the subscribe mode. Same socket as the
request side; the client just reads + filters events. --filter
SUBSYSTEM or SUBSYSTEM:ID narrows the stream. Exits cleanly on
SIGINT.

defaultSocketPath() now respects CLIDE_SOCKET_PATH before XDG — the
existing override callers always had this up their sleeve (via
XDG_RUNTIME_DIR manipulation) but making it explicit unblocks
parallel test runs where each test needs its own daemon socket. The
new end-to-end CLI suite does exactly that: 5 tests spin up real
daemon subprocesses and exercise the shortcut surface through the
live IPC stack.

74 core tests pass; round-trip verified by hand (open README.md →
insert → tail --events captures editor.opened / edited /
selection-changed / saved).

Co-Authored-By: Claude <noreply@anthropic.com>

lib/src/ipc/paths.dart

@@ -1,6 +1,19 @@
 import 'dart:io';
 
+/// Resolve the daemon unix-socket path.
+///
+/// Precedence (highest first):
+///   1. `CLIDE_SOCKET_PATH` — explicit override. Used by tests that
+///      run multiple daemons in parallel and by power users who want
+///      their own layout.
+///   2. `$XDG_RUNTIME_DIR/clide-<user>.sock` — Linux default; the
+///      per-user tmpfs lives exactly for this kind of short-lived
+///      socket and is auto-cleaned on logout.
+///   3. `/tmp/clide-<user>.sock` — fallback for environments without
+///      `XDG_RUNTIME_DIR`.
 String defaultSocketPath() {
+  final override = Platform.environment['CLIDE_SOCKET_PATH'];
+  if (override != null && override.isNotEmpty) return override;
   final xdg = Platform.environment['XDG_RUNTIME_DIR'];
   final user = Platform.environment['USER'] ?? 'anon';
   final base = (xdg != null && xdg.isNotEmpty) ? xdg : '/tmp';