engage: add -r flag to reload config of a running squadron

[mlund01]

Summary

• A second squadron engage against a config dir that already has a running instance now errors out with a clear hint, instead of forking and dying later in daemon.Fork's IsRunning check.
• New --reload / -r flag: signals the running daemon (SIGHUP) to re-read and validate its config. The reload goes through the same wsbridge.Client.ReloadConfig path the command center uses, so it inherits the validate-then-atomic-swap semantics — a broken config is rejected and the running daemon keeps its previous config.
• -r with no running daemon is a no-op: prints a message and proceeds with normal startup.

Behavior matrix

Running?	-r?	Result
no	no	normal startup
no	yes	prints Squadron is not running — ignoring -r and starting it now., then normal startup
yes	no	Error: squadron is already running (PID N). + suggestion to use -r or disengage, exits 1
yes	yes	reload flow (success → Config reloaded successfully.; failure → error printed, daemon keeps previous config)

What the user sees on -r

Success:

Squadron is already running (PID 12345). Reloading config from /path/to/config...
⠋ Validating and applying
Config reloaded successfully.

Failure (validation error in new HCL):

Squadron is already running (PID 12345). Reloading config from /path/to/config...
Config reload failed: <validation error>
Squadron is still running with the previous config (PID 12345). Fix the error above and re-run 'squadron engage'.

Implementation notes

• daemon.Reload(configPath) (int, error) sends SIGHUP to the PID recorded in engage.pid.
• Foreground engage path registers a SIGHUP handler that calls client.ReloadConfig() (existing function — same one the command center's TypeReloadConfig handler calls). On validation failure the handler writes SignalFailed to the ready file; on success the existing OnConfigLoaded callback writes SignalReady.
• Parent engage -r clears the ready file, sends SIGHUP via daemon.Reload, then WaitReadys for the daemon's result with a 30s timeout.

Test plan

• go build ./...
• go test ./internal/daemon/ ./cmd/
• New tests cover: missing PID file, malformed PID file, stale PID, actual SIGHUP delivery
• Manual: start squadron, run squadron engage → see already-running error
• Manual: edit HCL, run squadron engage -r → see reload succeed
• Manual: introduce HCL syntax error, run squadron engage -r → see error + confirmation old config still active
• Manual: squadron engage -r with nothing running → see "ignoring -r" message and normal startup