Replace generic setup boilerplate in CONTRIBUTING.md with ErrLens-specific instructions

[Copilot]

CONTRIBUTING.md was added with a placeholder "Set Up the Project" section containing generic npm install # or pip install -r requirements.txt copy-paste and the vague phrase "Follow the project-specific setup instructions". This replaces that block with concrete, ErrLens-specific steps.

Changes

• Prerequisites: Specifies Node.js v20.0.0+ (from package.json engines) and explicitly notes no Docker, .env, or database setup is required
• Install: npm install only — removes the irrelevant pip install alternative
• Verify: node bin/index.js --version to confirm the local CLI works after install
• Test: npm test with the underlying command (node --test test/**/*.test-lang.js) made visible
• No dev server: Adds a note clarifying ErrLens is a CLI tool and directs contributors to use node bin/index.js locally instead of the installed errlens binary

# Install dependencies using npm
npm install

# Verify the CLI works locally
node bin/index.js --version

# Run the test suite
npm test
# which runs: node --test test/**/*.test-lang.js

Original prompt

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This section details on the original issue you should resolve

<issue_title>Verify each finding against the current code and only fix it if needed.</issue_title>
<issue_description>Verify each finding against the current code and only fix it if needed.

In @CONTRIBUTING.md around lines 24 - 31, Replace the generic "Follow the
project-specific setup instructions" sentence and the generic bash example block
with concrete, repository-specific setup steps: list the exact package manager
commands to install dependencies (e.g., npm/yarn/pip commands used by this
project), any required global tooling or versions (Node/Python, Docker), how to
create or populate .env or config files, commands to run database migrations or
seed data (if applicable), and the exact command(s) to run tests and start the
dev server; update the section that currently contains the bash snippet so it
contains these precise commands and brief notes about prerequisites, and ensure
references to the existing phrasing "Follow the project-specific setup
instructions" and the bash example block are replaced accordingly.

Originally posted by @jaseel0 in #72 (comment)
</issue_description>

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

• Fixes Verify each finding against the current code and only fix it if needed. #73

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