fix: handle SIGINT/SIGTERM during initial generation

[JamBalaya56562]

Problem

Generate() runs the initial, synchronous generateFromContainers() before any of
the watch loops start:

g.generateFromContainers()   // synchronous; signal.Notify has not run yet
g.generateAtInterval()       // newSignalChannel() first runs inside these goroutines
g.generateFromEvents()
g.generateFromSignals()
g.wg.Wait()

signal.Notify is what suppresses the Go default disposition, and it was only ever
called from inside those goroutines. A SIGINT or SIGTERM arriving during the
initial generation therefore killed the process outright. The window is not
negligible — the initial pass makes one Docker API round trip per container.

cmd/docker-gen/main.go already guards this exact class of problem, but only for
SIGHUP:

// SIGHUP is used to trigger generation but go programs call os.Exit(2) at default.
// Ignore the signal until the handler is registered:
signal.Ignore(syscall.SIGHUP)

SIGINT and SIGTERM were left unprotected, which matches the exit status reported
in #201.

Change

• Register the signal channel before the initial generation, and check it once
  generation completes, so docker-gen finishes the in-flight generation and then
  exits gracefully.
• signal.Ignore is deliberately not used for SIGINT/SIGTERM. Ignoring them
  would make docker stop during startup hang until its timeout and then SIGKILL —
  worse than the current behaviour. They have to be received and handled.
• The channel is only registered when a config actually watches, mirroring the
  existing decision in generateFromSignals not to watch signals when there is
  nothing to watch. One-shot runs keep their current abort semantics.
• hasWatcher() is extracted from generateFromSignals so both call sites share it.

Implementation notes

The signal channel constructor is now injectable on the generator, following the same
pattern as GetCurrentContainerID. This makes the ordering testable without sending
real signals to the test process, which also keeps the test running on Windows rather
than being skipped.

Tests

TestGenerateStopsOnSignalDuringInitialGeneration covers SIGINT and SIGTERM. The
injected constructor records that it ran, and the /containers/json handler asserts
that flag was already set when the initial generation reached it — so the test fails
if registration ever moves back after generation. It also asserts /events is never
requested, confirming the watch loops do not start.

Scope

The other causes discussed in #201 are already resolved: the "is a directory" reports
were a bind-mount user error, and the SIGHUP race was fixed by the signal.Ignore
call above plus the graceful handler. This PR closes the remaining gap.

Fixes #201

🤖 Generated with Claude Code