feat: install.sh version gate — --force + per-commit 0.0cN

Architect-approved (C): install.sh now skips+aborts (exit 0) when the
installed version equals the current git-derived version, with a --force
flag to re-install. Version scheme 0.0cN (N = git commit count) bumps by
construction on every commit.

- install_version(): derives 0.0c$count via git rev-list; empty when .git
  absent (gate skipped); INSTALL_VERSION_OVERRIDE presence-check seam for
  tests (empty override simulates no-git deterministically)
- Gate after arg parse, before phases; numeric comparison (strip 0.0c, -eq);
  messages: 'Already installed (X). Use --force to re-install.' and
  '(dry-run) Would skip install: already at version X', both exit 0
- flag_set installed_version after 'Bootstrap complete' banner (not in
  dry-run, even under --force, never on phase failure)
- Docs: SCRIPTS.md flag table + gate description, AGENT_Context line count
  248->301 + flags + flow diagram, README --force row, tests/README row,
  AGENT_TODO Done entry

Verified: new tests/t-install-version.sh 21 checks (9 contracted cases,
real install.sh + hermetic env seams); suite 20 files / 461 checks /
0 fail / 0 skip; make gen byte-idempotent; make check OK; make lint
0 FAIL, 0 WARN; bash -n clean; git diff --check clean; Reviewer
APPROVE_WITH_NOTES with 3 mutation disproofs (8/21, 9/21, 4/21 fail)

tests/README.md

@@ -53,4 +53,5 @@ silently.
 | `t-uninstall-manifest.sh` | install.sh ↔ POS_LIBS symmetry, user-unit discovery, marker-driven plugin removal |
 | `t-gen-docs-drift.sh` | `make gen` idempotence on a pristine tracked tree (CI drift gate) |
 | `t-lint-gate.sh` | `make lint` green on the real tree; planted violations are caught and named |
+| `t-install-version.sh` | install.sh version gate: match→skip, mismatch→proceed, --force bypass, dry-run variant, flag write, numeric comparison |
 | `t-share-mountpoint.sh` | share-client `ask_mountpoint` UX: existing/new/declined/rejected paths, confirm gate, mkdir side effects, non-TTY stdin contract, static `n`→`t` guards |