docs: refresh README to reflect current addon state (#42)

[Xerrion]

Description

Refreshes README.md so it accurately reflects the addon as it sits on master. No source code changes - documentation only.

Highlights:

• New Configuration section documenting /phd config and the AceConfig options panel from PR feat: add AceConfig options UI for overlay positioning (#40) #41
• Updated Features list to mention the configurable action bar overlay
• Added /phd config row to the slash command table; fixed the /phd (no args) row description
• New Architecture section introducing the 4-layer Core / Engine / Data / Presentation split
• New Development section pointing contributors at luacheck and busted
• License link corrected from main to master

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Refactor (non-breaking change that improves code quality)
• Documentation update
• Breaking change (fix or feature that would cause existing functionality to change)

Related Issues

Closes #42

Testing

• Luacheck passes (luacheck .) - no code changed, still clean
• Tested in-game manually - N/A, docs only
• WoW version(s) tested on: N/A - documentation only

Verification performed:

• Visual review of rendered Markdown
• All tables align; no em/en dashes; only ASCII hyphens
• All claims verified against current source on master

Screenshots

N/A - Markdown documentation only.

Checklist

• My code follows the project's code style (4-space indent, 120 char lines)
• I have tested my changes in-game - N/A, docs only
• Luacheck reports no warnings - no code touched
• My commits follow conventional commit format

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 61f84dc7-962a-4f72-99ce-759a57aeeed1

📥 Commits

Reviewing files that changed from the base of the PR and between 690d9ce and 106c3f8.

📒 Files selected for processing (1)

• README.md

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