Fixes for AI docs (#58969) (cherry-pick to preview)

[zed-zippy]

Cherry-pick of #58969 to preview

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Audited the AI documentation against the implementation

Changes

• tool-permissions: skill tool patterns match the absolute
  SKILL.md path, not the skill name (fixed the table, example pattern,
  and added a clarifying note).
• external-agents: Gemini CLI receives Zed's Google AI key when no
  env key is set; extension-provided agents are deprecated in favor of the
  ACP registry.
• inline-assistant: alternatives run alongside the inline assistant
  model (not just the default model); fixed a broken anchor and
  gpt-4-mini → gpt-5-mini model id typos.
• mcp: corrected the extensions menu item name ("Install New
  Servers…"), the remote server url example, and removed the nonexistent
  thinking tool from the profile example.
• agent-panel: "New From Summary" lives in the New Thread menu;
  corrected the @-mention type list (no "instruction files"; added Fetch
  and Branch Diff).
• use-api-access: DeepSeek default api_url includes /v1.
• skills: folder name is a convention not a requirement; long
  descriptions warn rather than fail (measured in bytes); hidden skills
  are also @-mentionable; dropped the unenforced consecutive-hyphen
  rule; reconciled the no-remote-registry note with GitHub import.
• edit-prediction: alt-tab and alt-l are bound across platforms;
  predictions are throttled rather than fired on every keystroke.
• getting-started, windows-and-projects, parallel-agents: Panel
  Layout lives in the title-bar user menu, with
  workspace::UseAgenticLayout/UseClassicLayout action references.

Release Notes:

• N/A

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Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>