Add project documentation

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

README.md

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+# disk-checker
+
+Fast Ubuntu-friendly CLI for scanning folders, checking file sizes, hashing the first chunk of same-size files, and reporting possible duplicates plus symlinks, hard links, special files, and scan errors.
+
+## Install Rust on Ubuntu
+
+```bash
+sudo apt update
+sudo apt install -y build-essential curl
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+source "$HOME/.cargo/env"
+```
+
+## Build
+
+```bash
+cargo build --release
+```
+
+The binary will be at:
+
+```bash
+target/release/disk-checker
+```
+
+## Usage
+
+Scan the current directory:
+
+```bash
+disk-checker
+```
+
+Scan one or more paths:
+
+```bash
+disk-checker ~/Downloads /mnt/shared
+```
+
+Use JSON for scripts:
+
+```bash
+disk-checker ~/Downloads --json
+```
+
+Hash a larger first chunk before grouping possible duplicates:
+
+```bash
+disk-checker ~/Downloads --hash-bytes 8MiB
+```
+
+Follow symlinks while still reporting them separately:
+
+```bash
+disk-checker ~/Downloads --follow-links
+```
+
+Verify possible duplicates with a full-file hash pass:
+
+```bash
+disk-checker ~/Downloads --verify-full
+```
+
+Limit hashing workers:
+
+```bash
+disk-checker ~/Downloads --threads 4
+```
+
+## Notes
+
+- By default, duplicate results are **possible duplicates**: same file size plus same first `1MiB` BLAKE3 hash.
+- This is intentionally fast because it avoids reading whole files unless you pass `--verify-full`.
+- Symlinks are not followed by default to avoid surprises and cycles.
+- Hard link groups are reported separately because they are multiple paths to the same inode, not extra disk copies.
+- Hidden files and gitignored files are included; this is a disk scanner, not a source-code search tool.