Ship prebuilt binaries and a task-first README

[jserv]

Installing hackmd-mcp meant a Rust toolchain and a full build, and the README opened with build commands and environment variable edge cases before it said what the server is for. This adds prebuilt binaries in the style of frama-c-mcp and rewrites the README around why an agent benefits from HackMD access and a five-step quick start, with the detail moved to docs/.

Every push to main that passes the whole lane now replaces a rolling latest release with binaries for Linux x86_64, macOS arm64 and Windows x86_64 (the platforms the test matrix runs on), plus SHA256SUMS. The Linux binary is linked by cargo-zigbuild against glibc 2.17, so it runs on any glibc distribution from RHEL 7 on; a CI step reads the newest GLIBC symbol off the binary, because the smoke run on Ubuntu 24.04 cannot see it. The publish script checks every input before deleting the old release, aborts on any API status other than 200/404, and does nothing when its commit is no longer the head of main.

The docs also correct claims the old README overstated or got wrong: only body edits and folder changes are read back, the body hash guard is optional and not atomic, and Codex forwards only the variables listed in env_vars, so the previous Codex setup left the server without a token.

Verified locally: make check (276 passed) and clippy with -D warnings; actionlint, shellcheck and shfmt on the workflow and .ci/ scripts; a zig cross build whose newest GLIBC symbol is 2.17, with the floor check passing at 2.17 and failing at 2.16; --self-check with no token and a fresh state directory exits 0; both release scripts against a stubbed gh (missing assets, wrong asset set, a 500, a stale commit, a normal publish); every relative link and anchor in README.md and docs/. The build and release jobs themselves have not run on GitHub yet, so this PR's checks are their first run (release only fires after merge).

Left out: Linux arm64, a musl build, and per-commit immutable releases.

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Summary by cubic

Makes installing hackmd-mcp a download instead of a Rust build, and rewrites the README to lead with why an agent benefits from HackMD access before a five-step quick start.

Release and install

• Every push to main that passes CI replaces a rolling latest release with binaries for Linux x86_64, macOS arm64, and Windows x86_64, plus SHA256SUMS.
• The Linux binary is linked against glibc 2.17 via cargo-zigbuild, so it runs on any glibc distribution from RHEL 7 on.
• The publish script checks all inputs before deleting the old release, aborts on any API failure, and does nothing when the commit is no longer the head of main.

Documentation

• Moves configuration, client setup, tool workflows, and development notes into docs/, accessed from a shorter README.
• Fixes install steps that failed on RHEL-family and minimal Linux, and examples that relied on ~ expansion in JSON and TOML configs.
• Corrects earlier claims: only body edits and folder changes are read back, the body hash guard is optional and not atomic, and Codex forwards only the variables listed in env_vars.

^{Written for commit 03d25a5. Summary will update on new commits.}