docs: simplify personal-team setup section in CONTRIBUTING.md

[datlechin]

Summary

Tighten the personal-team Debug-build section added in #1028. Same instructions, ~40% fewer words, easier to scan.

Changes

• Drop the prose intro and fold context into the step list.
• Drop trivia that doesn't affect what a contributor types: the "developer portal reservation" note and the plugin-sub-ID note.
• Drop the "either revert or skip-worktree" alternative; just give the one that works (skip-worktree).
• Promote the section to ### Building with a personal Apple team so the heading shows up in nav.
• Remove em dashes (CLAUDE.md writing-style rule).

Test plan

• git diff --cached -U0 | grep -nE '—|seamless|robust|...' clean on added lines
• Markdown renders correctly (numbered list, code block, heading)

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