docs: extract goget manpage to standalone troff source

CHANGELOG.md

@@ -9,6 +9,25 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+**Documentation**
+
+- **Standalone troff manpage** — `man/goget.1` is now the single source
+  of truth for the goget(1) manual. The previous embedded manpage listed
+  only ~30 of the 138 CLI flags and was missing every standard section;
+  the new file documents every flag with a dedicated entry, the
+  full set of conventional `.SH` sections (NAME, SYNOPSIS, DESCRIPTION,
+  OPTIONS, EXAMPLES, EXIT STATUS, FILES, ENVIRONMENT, SEE ALSO, HISTORY,
+  AUTHORS, REPORTING BUGS, COPYRIGHT), curl/wget compatibility notes, and
+  a release-history entry. The manpage is embedded into the binary via
+  `//go:embed`, so `--generate-man-page` and `man goget` now agree.
+- **`just man`** — new task that runs `groff -man -ww -Kutf-8 -z` over
+  `man/goget.1` to validate the troff source. Skips cleanly when `groff`
+  is not installed (CI on minimal images still passes).
+- **`just install-man`** and **`just uninstall-man`** — install the
+  manpage to `$HOME/.local/share/man/man1/goget.1.gz` (XDG rootless,
+  gzipped, reinstall-safe). After `just install-man`, `man goget`
+  resolves the page automatically via the user's `MANPATH`.
+
 **Core engine**
 
 - **Unified worker pool** — single protocol-agnostic worker pool (`internal/pool`)
@@ -53,6 +72,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   output when combined with `--json`. Credentials embedded in the stored
   URL are masked before display (the password placeholder is `xxxxx`).
 
+### Testing
+
+- `cmd/goget/man_page_test.go` — new test file. Asserts the embedded
+  manpage is non-empty, carries a `.TH GOGET 1` header, contains every
+  mandatory `.SH` section, and documents every flag registered by
+  `defineFlags` (so adding a new flag without documenting it now breaks
+  the build). When `groff` is installed, also runs `groff -man -ww
+  -Kutf-8 -z` over the embedded source and fails on any warning.
+
 ## [1.0.0] — 2026-06-26
 
 First public release: a modern IPv6-first command-line download utility built on