fix(build): n8 build always builds the nemesis8 container

[kordless]

Problem

Two failure modes let n8 build build the wrong thing — or nothing:

1. cwd hijack. project_dir_fn() treated any Dockerfile in the current directory as the nemesis8 source. Running n8 build (or triggering an auto-build) from a sibling repo that ships a Dockerfile (e.g. skiff) compiled that project and stamped it nemesis8:latest (DEFAULT_IMAGE is fixed), silently clobbering the real agent image.

2. Installed users have no repo/Dockerfile. The fetch path (~/.nemesis8/project) shallow-cloned main via git, which (a) requires git on the user's machine and (b) floats to main instead of the tag matching the installed binary — so the built container could drift out of sync with the binary.

Fix

• cwd guard (is_nemesis8_dockerfile): the cwd Dockerfile is trusted only if it carries a nemesis8 build marker (nemesis8-base / nemesis8-entry). A foreign Dockerfile is ignored with a warning, and resolution falls through to the real nemesis8 source.
• Versioned, git-free fetch: download_build_files now downloads the GitHub source tarball for this binary's version (v{CARGO_PKG_VERSION}) over HTTPS (reqwest::blocking, already used in entry.rs) and unpacks it with flate2 + tar — no git required. A shallow git clone of the tag remains a fallback. The context is cached under ~/.nemesis8/project with a .nemesis8-version stamp and re-fetched only on version mismatch.

Net: n8 build always builds the nemesis8 container matching the binary — never a foreign project, never a floating main.

Verification

Compiled locally with cargo build (stable 1.97.1, x86_64-pc-windows-msvc) — Finished with no errors. flate2 + transitive deps locked in Cargo.lock.

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