chore: release v2.2.0 "Carbon and Nitrogen in Sync"

[Copilot]

Promotes the unreleased changes to v2.2.0, with the theme "Carbon and Nitrogen in Sync" reflecting that this release primarily stabilizes the broken C/N accounting introduced in v2.1.0 (PR #359, fixing #350 and #325), alongside new phenology and visualization features.

Changes

• docs/CHANGELOG.md — promotes [Unreleased] to ## SIPNET 2.2.0 - "Carbon-Nitrogen Refinement"; expands the previously empty ### Fixed section with C/N accounting corrections from PR SIP350 Update carbon accounting + plant mortality #359; fills in Git SHA (70762ca); opens a new empty [Unreleased] section
• CITATION.cff — bumps version to 2.2.0 in both the top-level and preferred-citation blocks
• src/sipnet/version.h — bumps NUMERIC_VERSION to "2.2.0"
• docs/Doxyfile — bumps PROJECT_NUMBER to 2.2.0

[dlebauer]

LGTM!

[Copilot]

Pull request overview

Promotes the repository metadata and documentation from v2.1.0 to v2.2.0, aligning version strings across the codebase, Doxygen docs, citation metadata, and release notes for the v2.2.0 release.

Changes:

• Bump SIPNET version to 2.2.0 in src/sipnet/version.h, docs/Doxyfile, and CITATION.cff.
• Promote the changelog’s [Unreleased] section into a SIPNET 2.2.0 release section and add the release Git SHA entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
docs/CHANGELOG.md	Creates the 2.2.0 release section, adds fixed items and a Git SHA, and reinitializes an empty [Unreleased] template.
CITATION.cff	Updates citation metadata version fields to 2.2.0.
src/sipnet/version.h	Updates the compiled-in numeric/version string to 2.2.0.
docs/Doxyfile	Updates Doxygen PROJECT_NUMBER to 2.2.0.

Suppressed comments (1)

docs/CHANGELOG.md:55

• These new changelog bullets end with periods, but the surrounding bullets do not. For consistency, remove the trailing periods.

- Corrected plant carbon accounting so the accounting-only carbon delta is not treated as nitrogen-bearing woody biomass (#359).
- Corrected nitrogen resorption and storage accounting during senescence and negative growth, including the ordering of storage use, fixation, and uptake (#359).
- Corrected the nitrogen mass-balance sign convention (#359).

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[dlebauer]

how do we make sure that this is up to date when we tag the release?