Harden scripts/deploy.sh strict-mode behavior and CLI validation

[VatsalSy]

Why

Recent review cycles showed recurring breakpoints in local deploy ergonomics and shell safety.

Evidence

• PR Harden content and script maintenance safeguards #62 (CodeRabbit review): strict mode (set -euo pipefail) can short-circuit fallback logic when find_available_livereload_port returns non-zero.
• PR 2025 wrap-up updates #61 (CodeRabbit review): --port / --host parsing should reject missing values and non-numeric ports.
• PR Harden content and script maintenance safeguards #62 (review suggestion): IPv6 host formatting should be bracketed in generated URLs.

Scope

• Refactor livereload-port assignment to a guarded branch (if LIVERELOAD_PORT=$(...)), not bare command substitution.
• Add explicit argument validation for -p|--port and --host (missing argument, flag-as-value, non-numeric port).
• Normalize URL rendering for IPv6 hosts by adding brackets when needed.
• Add a lightweight shell smoke-test script for these branches.

Acceptance Criteria

• scripts/deploy.sh continues running when no livereload port is available and disables livereload cleanly.
• Invalid --port and missing --host/--port args fail with clear messages and non-zero exit.
• IPv6 host URLs are emitted in valid bracketed form.
• Smoke test(s) cover the above paths and pass in CI/local.