Merge revision research into canonical plan

[ben]

(Ben:) I let an opus run with the full release-notes site and its training data, and it came up with more details about what needs to be updated for 3e.

Source documents

• 3rd-edition-audit.md — chapter-by-chapter ecosystem/tooling audit covering GitHub UI changes, hosting landscape, IDE tooling, and staleness items not tied to Git 3.0.
• git_features_2014_2024.md — chronological inventory of Git 2.0–2.47 feature changes used to cross-reference both planning docs.

What changed in REVISION_PLAN.md

Per-chapter additions

• Ch 1: Platform version cleanup (remove XP/Vista/Mavericks), id_dsa → Ed25519, Notepad++ removal, modern help resources
• Ch 2: git:// → legacy, git stash save → push, .mailmap default (2.23), :(attr:...) pathspec (2.44)
• Ch 3: git switch/switch -c as primary, ort merge strategy (2.34), rebase merge-backend note (2.26), modern workflows (GitHub Flow, trunk-based, stacked diffs), brief git worktree intro
• Ch 4: SSH key cleanup (id_dsa, -o caveat, authorized_keys2), xinetd/sysvinit removal, Apache 2.4 syntax fix, .htpasswd security note, hosting landscape expansion (Codeberg, Sourcehut, Azure DevOps), Git LFS section, partial clones section
• Ch 5: gh CLI, CI/CD integration, trunk-based development, git request-pull niche context, Gmail plaintext password security note
• Ch 6: Expanded from 3 bullets to 13 — WebAuthn/passkeys, basic auth removal → PATs/OAuth, GitHub Apps, Actions, GraphQL API, gh CLI, branch protection, CODEOWNERS, Dependabot, Discussions, Packages, Copilot/AI, Octokit expansion. Flags the structural decision: split into two chapters or expand as one.
• Ch 7: GPG key size (2048-bit → 4096/Ed25519), git bisect run, rebase --autosquash non-interactive (2.44), rebase auto-skip baseline (2.34), git stash save → push, submodule version-gate cleanup
• Ch 8: includeIf, core.hooksPath, modern hooks survey, XDG config prominence, url.insteadOf, committer config overrides (2.22)
• Ch 9: Forge-to-forge migration section, Perforce scope context, structural rename decision flagged
• Ch 10: Dumb protocol shrink note, Pack v2, commit-graph coverage, partial clone internals
• App A: Sublime Text trim note, GUI client survey update, Fish/Starship terminal integrations, AI-assisted Git tools category
• App B: gitoxide (Rust) section, libgit2 bindings expansion (Rust/Go/Python/Node)

Structural additions

• git merge-tree (2.38) added to new commands table
• Recommended new material items 8–9: git merge-tree, stacked diffs/PRs
• Step 0 added to Suggested Sequencing: resolve open structural decisions (Ch 6 split, Ch 9 rename) before writing begins
• New Effort by chapter summary table before Companion Documents