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, 9 insertions, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 3a99088..d293395 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,6 +2,15 @@
 resolver = "2"
 members = ["crates/gipfel-core", "crates/gipfel-cli", "crates/gipfel-app"]
 
+# Single source of truth for metadata inherited by every crate via
+# `version.workspace = true` etc. Bump `version` here on release; the
+# About dialog and all crates follow. A test enforces the ChangeLog top
+# entry matches this version.
+[workspace.package]
+version = "0.5.4"
+edition = "2021"
+license = "GPL-2.0-or-later"
+
 [workspace.dependencies]
 kamadak-exif = "0.6"
 quick-xml = { version = "0.41", default-features = false }