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).

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