feat: forward SIGINT/SIGTERM to wrapped tools for graceful shutdown

[gtsiolis]

Pressing Ctrl-C during a wrapped-tool run (lstk terraform apply, aws, cdk, sam, az, extensions) killed the child immediately instead of letting it shut down gracefully — so terraform never got to release its state lock. The tools run via exec.CommandContext with lstk's root context, which is cancelled on SIGINT/SIGTERM; exec.CommandContext's default Cancel then SIGKILLs the child right away.

All wrapped-tool passthroughs now run through a new proc.Run helper that:

• disarms the SIGKILL-on-cancel (its Cancel returns os.ErrProcessDone), so the tool terminates from the signal it received and lstk waits for its own shutdown — preserving the real exit code (Ctrl-C on terraform now exits 130 after full cleanup instead of signal: killed/255);
• forwards SIGTERM unconditionally: a terminal never generates it, so without forwarding kill <lstk-pid>, timeout 30 lstk …, or an IDE stop button would never reach the tool;
• forwards SIGINT only when none of stdin/stdout/stderr is a terminal: an attached terminal already delivers Ctrl-C to the child via the foreground process group, and a second near-simultaneous SIGINT makes tools like terraform abort without cleaning up. Checking all three streams matters — with only stdin redirected (yes | lstk terraform apply) the process group still delivers;
• registers its signal handler before Start, so a signal landing during startup is queued rather than lost.

(npm/launcher.js forwards unconditionally instead — safe there because its child is lstk itself, which tolerates duplicate signals; wrapped tools don't.)

Short internal captured-output execs (version checks, schema discovery, backend provisioning) are unchanged. No configurable grace period yet, per the issue scope.

Tests: internal/proc unit tests cover the disarm/exit-code semantics and the forwarding policy; test/integration/signal_forwarding_test.go covers the behavior end to end through a new signal-wait mode of the reference extension (exits 40 + number of signals received) — non-interactive SIGINT/SIGTERM forwarding, exactly-one SIGINT on Ctrl-C in a PTY, the piped-stdin double-signal regression, and SIGTERM reaching the tool from inside a terminal.

Fixes DEVX-1000