Add local guide for running and testing a local fork

[gaggle]

Summary

• What changed: Adds a "Running your fork locally" subsection under
  Contributing in README.md, walking through the two steps
  needed to make a clone of this repo serve as your daily-driver dev.
• Why: app.ts runs directly via its shebang, but two gotchas trip up
  contributors trying to test changes end-to-end: (1) symlinking app.ts
  breaks Deno's deno.json resolution because Deno doesn't follow symlinks
  to find its config file, so a thin shell wrapper is needed; (2) the
  shellcode embeds the dev binary path by searching $PATH at generation
  time, so the wrapper has to win that lookup over any other installed dev
  (e.g. from pkgm or Homebrew).

AI Intent

• Prompt or task statement: While debugging an unrelated issue, author wanted to run a local fork of dev to validate
  pkgxdev/dev#61 in their daily
  shell environment. The setup wasn't obvious from the existing README, so once
  worked out, the author asked the assistant to capture the procedure as a
  README addition so future contributors don't re-derive it.
• Scope of AI-assisted changes: Drafted the new subsection's prose,
  shell snippets, and the verification command. Author tested each step in
  their own environment and reviewed the final wording.

Risk Assessment

• Risk level: low, documentation-only change.
• Primary risks:
• Compatibility impact: None.

Rollback Plan

• Revert path: Revert the commit.
• Forward-fix path (if revert is not possible):

Validation

• Local checks run: Each shell snippet was executed by the author
  during the prior debugging session and produced the expected results
  (wrapper script created with correct content via the heredoc, hook body
  embedded the local path, grep eval returned the expected line). Markdown
  rendering reviewed locally.
• CI checks expected: The change should be CI-neutral.

Internal/Public Boundary Check

• No internal-only data, private URLs, secrets, or private runbooks were
  added.