Auto-select next free serve port when default (4096) is taken

[Copilot]

When --serve-port is omitted and port 4096 is already bound on the host, construct would silently fail when Docker tried to publish the port. This adds a best-effort port scan to pick the next free port and warn the user.

Behaviour

• Only applies when --serve-port is not specified (explicit ports are used as-is)
• Scans 127.0.0.1:<port> from 4096 upward, up to 100 ports
• Prints a yellow diagnostic to stderr if the default was unavailable:

  construct: port 4096 is already in use; using port 4097 instead
  

• Returns an error if the entire 100-port range is exhausted (uncommon; instructs user to pass --serve-port explicitly)

Changes

• internal/runner/runner.go — isPortFree(port) probes a loopback TCP bind; findFreePort(start) scans up to 100 ports and returns 0 on exhaustion; Run uses these when cfg.ServePort == 0
• internal/runner/runner_test.go — 5 new unit tests covering free/busy detection and range exhaustion
• docs/spec/serve-mode.md — documents the auto-fallback behaviour and 100-port search cap
• CHANGELOG.md — entry under [Unreleased]

Note: There is an inherent TOCTOU race between the port check and Docker binding, but it is negligible in practice — the probe runs immediately before container start.

Original prompt

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This section details on the original issue you should resolve

<issue_title>if we call serve without specified port and the port is taken, take the next free higher port.</issue_title>
<issue_description>Don't forget to check/update specs. And Changelog.

It should print that the port was taken and another port was chosen. Maybe yellow and in the error output.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes if we call serve without specified port and the port is taken, take the next free higher port. #6

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