maintenance: reconcile Rust MSRV documentation

[totocaster]

Affected components

• AGENTS.md ground rules
• README.md technology stack
• Cargo.toml workspace rust-version
• rust-toolchain.toml
• CI workflows under .github/workflows/

Risk

Repository guidance still says to build against Rust 1.86, while the workspace metadata, checked-in toolchain, README, and CI use Rust 1.88. This can confuse automation and reviewers about the real MSRV.

Impact

A stale MSRV policy can cause incompatible local validation, unclear review comments, or accidental reliance on a compiler version different from the one CI enforces.

Proposed fix

Decide whether Arrowhead officially requires Rust 1.88 or should restore Rust 1.86 compatibility. Then update the docs/toolchain/CI as one explicit policy change and add a quick MSRV verification step if applicable.

Suggested tests

• If keeping Rust 1.88: run the existing pre-flight commands on Rust 1.88 and update AGENTS.md.
• If restoring Rust 1.86: run cargo +1.86.0 check --all-targets, fix incompatibilities, and adjust Cargo.toml, rust-toolchain.toml, README, and CI.

Why not implemented automatically

This is a project policy decision, not a mechanical fix. The current codebase and CI already use Rust 1.88, but changing the stated MSRV should be intentional.