fix(run): confine dispatch guidance to the preparing host

[slowdini]

What this changes

The generated dispatch and judge recipes carry the absolute paths of the host that prepared the workspace, so the shell that eventually runs them has to resolve those same paths. Git Bash shares the Windows filesystem and does. WSL resolves its own namespace, where a C:\... path names nothing — and nothing in the tree translates between the two (no wslpath, no /mnt/c handling anywhere).

Guidance that listed Git Bash and WSL side by side as equivalent Windows options pointed at a split that fails quietly. src/cli/run/orchestrate/shell.rs went further and documented it as supported — "Preparing on Windows and dispatching from WSL is a legitimate split" — while its preflight warning told operators to "prepare here and dispatch them from a POSIX shell."

User-facing change

POSIX_TOOLING_REQUIREMENT (src/core/runtime.rs) states the constraint once and names WSL as where eval-magic runs rather than somewhere to dispatch into. Its four Markdown consumers — RUNBOOK.md, dispatch-manifest.md, and both shell-discovery errors — pick that up automatically. AFTER_HELP mirrors it by hand for clap, per the convention in AGENTS.md.

Preflight warning, before and after:

⚠ no POSIX shell found. ... The workspace and recipes below are still
  correct — prepare here and dispatch them from a POSIX shell.

⚠ no POSIX shell found. ... The workspace and recipes below are still
  correct — dispatch them from a POSIX shell on this host.

Documentation

docs/developer_overview.md gains a Platform support section recording the tiers: Linux and macOS supported, Windows through Git Bash deprecated with removal gated on #256, and the Windows-prepare/WSL-dispatch split unsupported with the reason it cannot work. AGENTS.md points at it; the README carries the user-facing deprecation note.

No schema or CLI-surface changes.

Verification

cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings
cargo test --all-targets      # 1146 passed, 0 failed

Two new tests, both written first and confirmed failing against the old wording:

• the_declared_requirement_places_wsl_around_eval_magic_not_downstream_of_it (src/core/runtime.rs)
• a_missing_shell_confines_dispatch_to_the_host_that_prepared_the_workspace (src/cli/run/orchestrate/shell.rs)

Ten golden fixtures re-blessed with GOLDEN_BLESS=1 cargo test golden_; the diff is one line per fixture — the requirement sentence and nothing else. cargo run -- --help checked for the rendered REQUIREMENTS block.

Context

This is the pre-refactor half of a Windows support decision: correct what is claimed now, and defer the code, CI, and distribution removal until after #256 replaces the POSIX recipe surface with a runner-driven eval-magic dispatch.

🤖 Generated with Claude Code