docs: restructure README, move detailed content to docs/ directory

docs/HASHLINE.md

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+# The Hashline Technique
+
+Hashline is AnchorCli's unique approach to safe, precise file editing.
+
+## How It Works
+
+Every line returned by file tools is prefixed with a content-derived hash anchor:
+
+```
+  function hello() {
+    return "world";
+  }
+```
+
+The format is `lineNumber:hash|content` where:
+- `lineNumber` is the current line number in the file
+- `hash` is an Adler-8 checksum derived from the line content and position
+- `content` is the actual line text
+
+## Editing with Anchors
+
+When editing, you reference these `line:hash` anchors instead of reproducing old content. Before any mutation, both the line number **and** hash are validated — stale anchors are rejected immediately.
+
+### Example
+
+To replace lines 2-3 from the example above:
+
+```json
+{
+  "tool": "replace_lines",
+  "path": "example.js",
+  "startAnchor": "2:f1",
+  "endAnchor": "3:0e",
+  "newLines": ["    return \"hello world\";"]
+}
+```
+
+## Benefits
+
+This eliminates:
+
+- **Whitespace/indentation reproduction errors** — No need to match exact spacing
+- **Silent edits to the wrong line** — Hash validation ensures you're editing the right content
+- **Entire-file rewrites** — Change just one line without touching the rest
+- **Line drift in batch operations** — BatchEdit validates all anchors upfront and applies changes bottom-to-top
+
+## Key Principles
+
+1. **Never include anchors in your content** — When using `replace_lines`, `insert_after`, or similar tools, the `newLines` parameter should contain raw source code only, WITHOUT the `lineNumber:hash|` prefix
+2. **Always validate before editing** — Re-read files to get fresh anchors if your previous anchors fail validation
+3. **Use BatchEdit for multiple changes** — This validates all anchors upfront and applies operations atomically
+
+---
+
+*Back to [README.md](../README.md)* | *See [TOOLS.md](TOOLS.md) for all available tools*