docs: fix mobilecli skill inaccuracies and document it in README

[gmegidish]

Summary

Review of skills/mobilecli/SKILL.md (added in #256) against actual CLI behavior turned up a few inaccuracies, now fixed:

• Quick Start used a device name (--device "iPhone 15"); --device takes a device id (serial/udid). Now lists devices first and uses <device-id>.
• allowed-tools frontmatter was space-separated and included npx; now just Bash(mobilecli:*). Removed the npx section.
• io button now documents the full button set, including Android-only BACK, ENTER, BACKSPACE, APP_SWITCH, and DPAD_* (per devices/android.go).
• Added agent status next to agent install in the iOS real-device notes.
• Replaced LaTeX center-point math with plain centerX = x + width/2.
• Removed ToC, anchor tags, and emoji headers — the file is consumed linearly by agents.

Also adds a Claude Code Skill section to README.md so users can discover the skill, with npx skills add mobile-next/mobilecli install instructions (verified working against this repo).

Test plan

• Verified every documented command/flag against mobilecli --help and subcommand help output
• Verified button keymap against devices/android.go
• Verified npx skills add mobile-next/mobilecli --list discovers the skill
• make lint failures are pre-existing on main (markdown-only diff)

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 7e981334-4888-48ac-847b-74c5698a51aa

📥 Commits

Reviewing files that changed from the base of the PR and between e1fdabb and 7995d85.

📒 Files selected for processing (2)

• README.md
• skills/mobilecli/SKILL.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/mobilecli-skill

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