docs(agent): harden instructions to author deliverables as merged and prohibit administrative tracking PRs

[dan-petty]

Description & Rationale

Harden AGENTS.md with foundational instructions:

1. Author All Code, Documentation, Status & Wording as Merged/Committed: All code, documentation, checklists, deliverable descriptions, and task tracking files must be written from the outset in their final completed state as they will read once committed and merged.
2. Strict Prohibition of Standalone Tracking Commits & Follow-Up Status PRs: Strictly prohibit creating separate commits, topic branches, or pull requests solely to update task statuses post-merge.
3. Strict Prohibition of Inconsistent N-Gate Numbering & Unified Gated Terminology: Enforce the unified Gated convention project-wide.