chore: simplify Premove agent routing

[aryamantodkar]

Why

The old Premove ITN routing put routine implementation behind a serial root-to-Implementer handoff. The root now owns dependent coding work. Named specialists remain available when separate context helps.

Changes

• Pin the root to GPT-6 Sol Medium, remove Implementer, and cap concurrent child agents at two.
• Align the generic agent configuration, plan authority, and routing policy with Premove PR #107.
• Remove default subagent model settings and use only the three named roles.
• Copy the Explorer and Architect prompts from Premove. Apply the same review format and scope to Reviewer while retaining Premove ITN's Rust/Python, candidate, scoring, decoder, public-result, and frozen-prediction checks.
• Document direct root work, bounded child tasks, minimal inherited context, and compact specialist reports.

Premove ITN boundaries

The ITN-specific architecture scope in AGENTS.md, runtime ownership rules, frozen evaluation policy, CodeGraph and RTK setup, validation guidance, and release instructions remain specific to this repository. No inference or normalization code changed.

Validation

• Compared the current Premove PR head with this branch: generic plan authority and routing match exactly after excluding the ITN-specific architecture paragraph; root and named-agent configuration match; Explorer and Architect role files match.
• Parsed all local agent TOML files with Python tomllib.
• git diff --check passed.
• GitHub Actions is the authoritative pull-request gate.