build: single source of truth for the version

Version now lives only in [workspace.package] in the root Cargo.toml;
all three crates inherit it via version.workspace = true (this also fixes
gipfel-core, which had drifted to 0.1.0 while the others were 0.5.4).
edition/license are inherited the same way.

The About dialog now reads env!("CARGO_PKG_VERSION") instead of a
hardcoded string. A new test (changelog_top_matches_version) asserts the
top gipfel-X.Y.Z entry in ChangeLog equals the crate version, so the
manifest, the About dialog and the ChangeLog cannot drift apart.

AGENTS.md gains a "Releasing a new version" section documenting the flow
(bump workspace version, add ChangeLog entry, tag, git archive source
tarball).

1 files changed, 29 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
index f8992c7..29a4029 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -123,6 +123,35 @@ models. If you regenerate, re-run the golden tests immediately.
   `dirs` + exe-relative) and font fallback, then get libcosmic itself to compile
   for `x86_64-pc-windows-gnu` (upstream does not test Windows).
 
+## Releasing a new version
+
+The version has a **single source of truth**: `[workspace.package] version` in
+the root `Cargo.toml`. All three crates inherit it (`version.workspace = true`),
+and the app's **About dialog** reads it at compile time via
+`env!("CARGO_PKG_VERSION")`. Do **not** hardcode a version anywhere else.
+
+To cut a release `X.Y.Z`:
+
+1. Bump `version` in `[workspace.package]` (root `Cargo.toml`). That updates all
+   three crates and the About dialog at once.
+2. Add a new entry at the **top** of `ChangeLog`:
+   ```
+   gipfel-X.Y.Z
+   * one line per user-visible change
+   ```
+   The test `gipfel-app: changelog_top_matches_version` fails if the top
+   `gipfel-X.Y.Z` line does not equal the crate version, so the manifest and the
+   ChangeLog cannot drift apart.
+3. `cargo test --lib --test golden` (with `GIPFEL_DATA` set) — must be green.
+4. Commit, then tag: `git tag vX.Y.Z`.
+5. Build the **source** tarball (tracked files only, no build artifacts):
+   ```sh
+   git archive --format=tar.gz --prefix=gipfel-X.Y.Z/ \
+       -o ~/gipfel-X.Y.Z.tar.gz vX.Y.Z
+   ```
+   Recipients build with `cargo build --release`. For a fully offline tarball,
+   run `cargo vendor --versioned-dirs vendor/` and commit `vendor/` first.
+
 ## Do / Don't
 
 - ✅ Run `cargo test --lib --test golden` (with `GIPFEL_DATA` set) after any