docs: add git-native KB ADR context

docs/adr/0001-git-native-knowledge-base.md

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+# ADR 0001: Git-native Knowledge Base (Git as source of truth, Postgres as derived index)
+
+- **Status:** Proposed (brainstorm outcome — for team review)
+- **Date:** 2026-07-24
+- **Origin:** Rohan Verma's meeting proposal to pivot from the custom-built KB "file system" to a Git-based system due to persistent maintenance issues; Thierry Bakera to investigate.
+
+---
+
+## Context
+
+### What we have today
+
+SurfSense does **not** actually have a file system. It has a **virtual filesystem façade mapped onto Postgres rows**, used by the chat agent. The moving parts:
+
+- Virtual `/documents/` namespace computed from DB rows — `surfsense_backend/app/agents/chat/runtime/path_resolver.py`
+- Read-side backend faking `ls`/`read`/`glob`/`grep` over Postgres — `.../filesystem/backends/kb_postgres.py`
+- Write-side "commit at end of turn" layer — `.../main_agent/middleware/kb_persistence/middleware.py`
+- **Three separate hand-rolled versioning/audit systems:**
+  - `DocumentVersion` (user history) — `app/utils/document_versioning.py`
+  - `DocumentRevision` / `FolderRevision` (agent revert snapshots) — `app/services/revert_service.py`
+  - `AgentActionLog` (tool-call audit)
+- Supporting machinery: fractional indexing for ordering, move tracking, `content_hash` change detection, chunk reconciliation.
+
+### The problem
+
+The team has been hand-implementing — on top of a relational DB never designed for it — the exact primitives Git provides natively. That re-implementation is the source of the "persistent maintenance issues."
+
+| Hand-rolled today | Git provides natively |
+|---|---|
+| `path_resolver` + folder tree | tree objects |
+| end-of-turn staged commit | atomic commits |
+| `DocumentVersion` snapshots | commit history |
+| `DocumentRevision` + `revert_service` | `git revert` / `reset` |
+| `AgentActionLog` | commit log / `blame` |
+| `content_hash` dedup | content-addressed blobs (SHA) |
+| fractional indexing / move tracking | tree diff / rename detection |
+
+### Search stack (unchanged by this ADR)
+
+Hybrid chunk search: pgvector (HNSW) + Postgres FTS + RRF, optional reranking. Chunking via Chonkie; incremental via `chunk_reconciler.py`. See `.../shared/retrieval/hybrid_search.py`.
+