Set up Linux development environment

[brainx]

Summary

The repo's documented build path (scripts/build-macos.sh, make build, CI) targets macOS/Homebrew, but the cloud dev VM is Linux. This PR adds a Linux build path and developer documentation so the project builds, runs, and tests on Linux without modifying any vintage source.

• scripts/build-linux.sh — mirrors build-macos.sh using system libraries (libssl-dev, libpcre3-dev (PCRE1), libncurses-dev, zlib1g-dev). Adds the -fcommon flag required by modern GCC/Clang for the legacy global declarations, generates Src/Makefile from Makefile.in, builds, and installs bin/mixmaster + bin/mixremailer.
• AGENTS.md — ## Cursor Cloud specific instructions documenting the Linux build, run/test commands, and non-obvious runtime gotchas (no MTA on VM, pool batching, self-addressed loopback, expired 2007 sample keys).

No existing code was changed.

Verification

Built both binaries and ran a full end-to-end anonymous-remailer flow: client encrypts a message → remailer decrypts the hop → final mail delivered with the recovered plaintext and anonymizing disclaimer.

mixmaster_e2e_demo.mp4

Mixmaster ncurses client menu

Delivered anonymous mail (final hop output):

final_delivered_mail.eml

• ./scripts/build-linux.sh — builds bin/mixmaster + bin/mixremailer
• MIXPATH="$PWD/Mix" ./bin/mixremailer -M — CI smoke test, exit 0 (auto-generates RSA key)
• cc -DHAVE_SETENV -include string.h Src/tests/test-parse_yearmonthday.c && ./a.out — prints OK.

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