docs: prune obsolete phase-planning docs + gitignore manifesto/

Remove four phase-planning docs superseded by the recent sediment work
(AUDIT_PHASE_B, EXTERNAL_INTEGRATION_PLAN, MATHLIB_METRIC_MIGRATION,
PHASE_A2_BOCHNER). Clean a stale cross-reference to them in
CONTRIBUTING.md, OpenGALib.lean module docstring, and
docs/NAMING_CONVENTION.md. Add `manifesto/` to .gitignore (lives in a
separate Overleaf project, not part of lib history).

Author: Xinze-Li-Moqian

.gitignore

@@ -35,3 +35,6 @@ lake-packages/
 # never enter our git history.
 external/
 
+# Charter manifesto (kept in separate Overleaf project, not part of lib history)
+manifesto/
+

CONTRIBUTING.md

@@ -37,9 +37,7 @@ merged contributions but liberally relaxable for sketches / WIP / drafts.
    `Tensor/`.
 4. **External references** — `external/` (git-ignored) contains the
    `qinz1yang/differential-geometry` reference repo we draw from.
-   See `docs/EXTERNAL_INTEGRATION_PLAN.md` for what's been re-implemented
-   here, what's planned, and what's skipped. We **re-implement** in our
-   conventions — never copy.
+   We **re-implement** in our conventions — never copy.
 
 ## PR workflow
 
@@ -87,8 +85,7 @@ mis-organized.
   (we strip on release).
 * `Inspired by <repo>` / `Adapted from <author>` attribution in
   source files — attribution belongs in commit history, not in
-  code or docstrings (we own our content per
-  `docs/EXTERNAL_INTEGRATION_PLAN.md`).
+  code or docstrings.
 * `paper §X` / project-specific cross-references in `Algebraic/`,
   `Tensor/`, `Riemannian/`, `GeometricMeasureTheory/` — these
   namespaces must be paper-agnostic; paper-specific work lives in

OpenGALib.lean

@@ -34,19 +34,6 @@ as a separate sub-project (`require OpenGALib from ".."`).
 * `GeometricMeasureTheory`  — finite-perimeter, varifolds, stationary,
                               tangent cones, rectifiability, isoperimetric.
 
-## Phase status
-
-* **Phase A (port)** — complete. ~10 000 lines of tensor / shuffle /
-  differential-form / operator content ported from external reference
-  (qinz1yang/differential-geometry). See `docs/EXTERNAL_INTEGRATION_PLAN.md`.
-* **Phase B (audit)** — complete. Findings in `docs/AUDIT_PHASE_B.md`. Net:
-  ported content is self-contained, currently isolated from lib core; bridges
-  to Curvature / Hessian-symmetry / manifold-DifferentialForm are tier-3
-  Phase C work, consumer-driven.
-* **Phase C (consolidation)** — in progress. Tier 1 (documentation / facade)
-  current; tier 2 (naming / `@[simp]` audit) deferred; tier 3 (substantive
-  bridges) consumer-driven.
-
 ## Sorry status
 
 Per `docs/SORRY_CATALOG.md`. The Riemannian package carries zero existence