Remove the old M1 docs and update AGENTS.md to match the request lifecycle

[luchobonatti]

User story / Problem statement

Currently docs/roadmap-m1.md and docs/m1-brief.md describe a synchronous 1:1 vault — deposit and withdraw priced at call time — that the current architecture replaced with a request lifecycle priced against attestations. Both were still cited as authoritative: CLAUDE.md named the roadmap as where decisions live, and AGENTS.md linked both. An agent or a new contributor reading either was pointed at a design that no longer holds.

AGENTS.md also taught the old model in its own body, described the repo as bootstrap-only after the contract crates had landed, and documented cargo test -p vault against a crate that does not exist.

Expected outcome

The documentation reflects the current design and nothing points to the old one.

Acceptance criteria

• docs/roadmap-m1.md and docs/m1-brief.md are removed
• no file references either path
• AGENTS.md describes the request lifecycle: pending, priced, claimed
• AGENTS.md states the real repo contents and that the vault contract is not written yet
• the documented test command works
• gotchas that no longer apply are dropped

Technical notes

Dependency versions are read from Cargo.toml, rust-toolchain.toml and flake.nix. The architecture is maintained outside this repository, so neither file links to a replacement document.

[luchobonatti]

Scope note: the technical note above said the architecture was maintained outside this repository and that neither file would link to a replacement. That changed while the work was in progress — the architecture is now committed as docs/architecture.md, and CLAUDE.md and AGENTS.md point at it. Recorded in the Deviations section of #80.