Merge revision research into canonical plan

Integrates two supplementary research documents into REVISION_PLAN.md:
- 3rd-edition-audit.md: chapter-by-chapter ecosystem/tooling audit
- git_features_2014_2024.md: Git 2.0–2.47 feature chronology

Key additions per-chapter:
- Ch 1: platform version cleanup, id_dsa removal, modern help resources
- Ch 2: git:// legacy, stash push, .mailmap default, :(attr:) pathspec
- Ch 3: git switch primary, ort strategy, rebase backend, modern workflows, worktree intro
- Ch 4: SSH key cleanup, Apache 2.4, LFS section, hosting landscape expansion
- Ch 5: gh CLI, CI/CD integration, trunk-based, Gmail security note
- Ch 6: 13-bullet expansion (Actions, Apps, gh CLI, passkeys, branch protection, etc.)
- Ch 7: GPG key size, bisect run, autosquash non-interactive, stash push, submodule cleanup
- Ch 8: includeIf, core.hooksPath, XDG prominence, url.insteadOf, committer overrides
- Ch 9: forge migration section, Perforce scope, structural rename decision
- Ch 10: dumb protocol shrink, Pack v2, commit-graph, partial clone internals
- App A: Fish/Starship, AI tools, GUI survey update
- App B: gitoxide, libgit2 bindings expansion

Also adds:
- git merge-tree to new commands table
- Recommended items 8-9 (merge-tree, stacked PRs)
- Step 0 in sequencing for open structural decisions (Ch 6 split, Ch 9 rename)
- Effort by chapter summary table

Author: ben