docs: add README with security model and CLAUDE.md project context

CLAUDE.md

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+# CLAUDE.md — idfoto
+
+## What is this
+
+idfoto is a git-backed, self-hostable password manager with a Rust core. Two-factor vault decryption: passphrase + a reference JPEG carrying a 256-bit secret embedded via DCT steganography. The server only ever sees opaque ciphertext.
+
+## Build and test
+
+```bash
+cargo build                    # build everything
+cargo test                     # run all tests (unit + integration)
+cargo test -p idfoto-core      # core library tests only
+cargo run -- --help            # CLI help
+cargo run -- generate -l 32   # quick smoke test
+```
+
+## Project structure
+
+```
+crates/
+├── idfoto-core/        # Platform-agnostic library (no filesystem, no git, no network)
+│   ├── src/
+│   │   ├── lib.rs      # Re-exports public API
+│   │   ├── error.rs    # IdfotoError enum (thiserror)
+│   │   ├── crypto.rs   # Argon2id KDF + XChaCha20-Poly1305 encrypt/decrypt
+│   │   ├── entry.rs    # Entry, ManifestEntry, Manifest structs (serde)
+│   │   ├── vault.rs    # encrypt_entry, decrypt_entry, encrypt_manifest, decrypt_manifest
+│   │   └── imgsecret.rs # DCT-based 256-bit secret embedding in JPEGs
+│   └── tests/
+│       └── integration.rs  # Full-workflow and two-factor independence tests
+└── idfoto-cli/         # CLI binary
+    └── src/
+        └── main.rs     # clap CLI: init, add, get, list, edit, rm, sync, generate, device
+```
+
+## Key design decisions
+
+- **idfoto-core is bytes-in/bytes-out.** No filesystem, no network, no git operations. Makes it portable to WASM, Android, iOS.
+- **XChaCha20-Poly1305** over AES-GCM — 192-bit nonce eliminates collision risk, fast in WASM/ARM without AES-NI.
+- **Single master_key** (no per-entry subkeys) — simpler, sufficient for family vault sizes.
+- **imgsecret uses central-embed DCT** — embeds only in the middle 70% of the image (15% crumple zone for crop tolerance), with majority voting across 5-50 redundant copies.
+- **QUANT_STEP = 50.0** — higher than typical (25) to survive JPEG recompression down to Q85.
+- **Device ed25519 keys are separate from the KDF.** Revoking a device doesn't require rotating the passphrase or reference image.
+
+## Crypto pipeline
+
+```
+passphrase (UTF-8 bytes) || image_secret (32 bytes from reference JPEG)
+  → Argon2id(salt=vault_salt, m=64MiB, t=3, p=4)
+  → master_key (32 bytes)
+  → XChaCha20-Poly1305(nonce=random 24 bytes)
+  → encrypted entry/manifest
+```
+
+## Conventions
+
+- Tests use fast Argon2id params (m=256, t=1, p=1) so they don't take forever.
+- Test JPEGs are generated synthetically via `make_test_jpeg()` — no binary test fixtures.
+- Entry IDs are random 8-char hex strings.
+- Git history is preserved as an audit log — no squashing.
+- The CLI shells out to `git` for sync — no libgit2/gitoxide dependency.
+
+## Remote
+
+Source code: `ssh://git@git.adlee.work:2222/alee/idfoto.git`
+
+## Design spec
+
+Full threat model, entropy analysis, and architecture: `docs/superpowers/specs/2026-04-11-idfoto-design.md`
+
+## Roadmap
+
+Next: WASM build + Chrome MV3 browser extension (Plan 2). Then mobile (Rust core compiles to ARM).