docs: reconcile '--gitignore-global' wording now that --visibility=global-default ships the capability

[Lillevang]

Problem

CLAUDE.md / .agent/AGENTS.md still describe a planned, not-yet-exposed --gitignore-global flag ("Global gitignore support is planned but not exposed until implemented", "Do not expose --gitignore-global until it is implemented and tested"). That capability has since shipped under a different surface: agent-init init --visibility=global-default (#52, PR #61), which writes a managed block to the machine-wide global excludes file with the idempotent managed-block + machine-wide-warning behavior the planned flag specified.

The canonical instructions now contradict the implemented CLI.

Why it matters

.agent/AGENTS.md (symlinked as CLAUDE.md) is the source of truth agents read before working in this repo. Describing --gitignore-global as unimplemented/forbidden while --visibility=global-default already delivers it will mislead future contributors and agents, and risks someone re-implementing it under the old flag name.

Acceptance criteria

• Reconcile the --gitignore-global references in .agent/AGENTS.md (and any in CLAUDE.md/docs) to describe the shipped --visibility=global-default mode instead, or explicitly note --gitignore-global was superseded by it.
• Keep the machine-wide-footgun warnings — they still apply, just under the new flag name.
• Verify docs/cli.md and README describe --visibility=global-default consistently (PR feat(cli): support --visibility=global-default machine-wide ignore (#52) #61 already adds these; confirm no stale --gitignore-global mention remains).

Notes

Touches this repo's own .agent/ canonical instructions, which are normally off-limits to edit without explicit ask — this issue is that explicit authorization once it's picked up. Best done after #61 merges.