feat(security): optional encryption at rest for engram.db

[GravityZenAI]

Feature Description

~/.engram/engram.db is a plaintext SQLite database. Anyone with filesystem access can read all stored decisions, architectural secrets, API patterns, bug details, and potentially sensitive context. This is a concern for:

• Shared machines (offices, labs, pair programming stations)
• Git sync — pushing engram.db to a repo exposes everything in commit history
• Laptop theft/loss — full disk encryption isn't always enabled
• Compliance — some organizations require encryption at rest for developer tools

Proposal

Optional encryption at rest using SQLCipher (drop-in SQLite replacement) or application-level AES-256 encryption.

Configuration:

# engram.toml
[security]
encrypt = true
key_source = "keyring"  # options: "keyring", "env:ENGRAM_KEY", "prompt"

Implementation options (in order of preference):

1. SQLCipher — transparent, encrypts the entire database file. Go bindings exist (github.com/mutecomm/go-sqlcipher). Most robust.
2. Application-level encryption — encrypt content and title fields before writing, decrypt on read. Simpler to implement, but FTS5 can't index encrypted content (would need a separate plaintext FTS index that's rebuilt on startup).
3. Hybrid — encrypt the entire DB with SQLCipher for storage, but this is the recommended approach.

Key management:

• Keyring (default): OS credential store (Windows Credential Manager, macOS Keychain, Linux Secret Service)
• Environment variable: ENGRAM_KEY=... for CI/automated environments
• Interactive prompt: Asked once per session, kept in memory

Migration path:

• engram encrypt — encrypts an existing plaintext database (creates backup first)
• engram decrypt — decrypts back to plaintext (for migration/debugging)
• New installations with encrypt = true start encrypted from day one

Design principles

• Opt-in: Zero cost when disabled. Default remains plaintext for simplicity
• Transparent: Once configured, all MCP tools, CLI, and TUI work identically
• Recoverable: Backup before encryption, decrypt command available
• Git sync compatible: Encrypted DB syncs normally (opaque blob)

Affected Area

Store (SQLite driver), CLI (new commands), Configuration