docs: document context diff command in README, getting-started, and history docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -27,6 +27,7 @@ When working with AI assistants on your Obsidian vault, you constantly re-explai
 - **Prompt templates** with placeholders and conditionals for reusable workflows
 - **Additional context sources** from freetext, external files, or shell commands
 - **Frontmatter presets** to configure context per-note via `ai-context` YAML
+- **Context diff** to copy only new and modified files since the last export
 - **Context snapshots** for saving and replaying context recipes
 - **Granular section selection** to include only the headings you need
 - **Context history** with diff, search, and one-click restore

docs/getting-started.md

@@ -77,6 +77,7 @@ The context is now in your clipboard. Paste it into Claude, ChatGPT, or any AI a
 | **Copy context (select sections)** | Choose specific headings |
 | **Copy context from frontmatter preset** | Use note's ai-context config |
 | **Copy smart context** | Auto-detect related notes via links, tags, and properties |
+| **Copy context diff** | Copy only files changed since last export |
 | **Export context (multi-target)** | One-click export using the active export profile |
 | **Generate context files** | Open the generator |
 | **View context history** | Browse past contexts |

docs/history.md

@@ -29,6 +29,40 @@ Ctrl+P > "Promptfire: View context history"
 
 Browse, search, compare, and restore any previous context.
 
+## Context Diff
+
+In iterative LLM workflows you don't want to paste the full vault context every time — only the files that changed. The context diff command compares per-file content hashes against the most recent export and copies only the delta.
+
+### Usage
+
+```
+Ctrl+P > "Promptfire: Copy context diff (changes since last export)"
+```
+
+### How It Works
+
+1. Every normal export stores a content hash per file in the history entry
+2. The diff command finds the most recent history entry with hashes (the "baseline")
+3. Current files are hashed and compared against the baseline
+4. Only new and modified files are copied, tagged with `[NEW]` or `[MODIFIED]` in the file header
+5. The diff entry stores all current hashes, so it becomes the next baseline — enabling chained iterative diffs
+
+Sources (freetext, file, shell) are always included fully since they're external.
+
+### Requirements
+
+- History must be enabled (Settings > History > Enabled)
+- At least one prior export with file hashes must exist (any normal copy after the update)
+
+### Edge Cases
+
+| Scenario | Behavior |
+|----------|----------|
+| No baseline exists | Notice: "Run a normal context copy first" |
+| No changes detected | Notice: "No changes since last export" |
+| File renamed | Shows as new file + removed file |
+| File deleted | Listed in removed count in the notice |
+
 ## Snapshots
 
 Snapshots save a context "recipe" — the exact combination of notes, settings, and template — so you can replay it later.