feat(init): default VS Code chat sessions to Squad #1486

Summary

Change squad init so it sets the repository's VS Code workspace default chat mode to Squad by default, via .vscode/settings.json:

{
  "chat.newSession.defaultMode": "Squad"
}

Behavior / Acceptance Criteria

• Plain squad init enables this behavior by default.
• If .vscode/settings.json does not exist, create it (and .vscode/ if needed) with "chat.newSession.defaultMode": "Squad".
• If .vscode/settings.json exists and chat.newSession.defaultMode is absent, add that property while preserving all existing settings.
• If chat.newSession.defaultMode already exists with any value, preserve it exactly and do not overwrite it (treat as user ownership, regardless of casing/value).
• Provide an explicit CLI opt-out following the repo's established --no-flag convention (e.g.--no-vscode-default). Document the exact flag in squad init --help` and relevant init documentation.
• With the opt-out flag, do not create or modify .vscode/settings.json for this behavior at all.
• Editing must be JSONC-aware: preserve comments, trailing commas, existing keys, and reasonable formatting. Reuse an existing helper/dependency where possible rather than hand-rolling a parser; do not replace the whole file or strip comments.
• If an existing settings file cannot be safely parsed/edited, do not corrupt or replace it - surface a warning consistent with existing CLI patterns and continue init (unless repo conventions require a hard failure).
• The operation must be idempotent: rerunning squad init causes no further diff.
• Scope is limited to squad init; this behavior should not be added to squad upgrade unless architecture makes init/upgrade inseparable (if so, that must be documented and justified).

Testing

Add/update focused tests covering:

• Missing settings file
• Existing settings file without the key
• Existing key present with a different value (must be preserved)
• Comments / trailing commas preserved after edit
• Opt-out flag behavior
• Idempotent rerun (no diff on second run)
• Malformed/unparseable existing file (safety - no corruption)

Docs

Update user-facing docs/help directly related to the new flag and default behavior.

Non-goals

• Do not modify unrelated files or pre-existing user work.
• Do not touch squad upgrade behavior unless justified per above.

Requested by Tamir Dresher.