Implement secure password generation (#8)

docs/password-generation.md

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+# Secure password generation
+
+`PasswordGenerator` owns `pass generate` semantics in `crates/storage`.
+Production calls use `OsRng`; tests inject a deterministic cryptographic RNG.
+Random bytes are converted to character indexes with fallible rejection
+sampling, so arbitrary set sizes have no modulo bias and operating-system RNG
+failure is reported before repository mutation.
+
+The default is 25 characters from printable ASCII punctuation and
+alphanumerics. `--no-symbols` selects ASCII letters and digits. A
+`GeneratorConfig` may supply another Unicode character set and default length.
+Lengths must be 1 through 4096. Sets must be nonempty and contain no duplicate
+or control characters; rejecting duplicates prevents accidental weighting.
+
+Normal generation uses the same overwrite decision and `--force` rules as
+insert. `--in-place` requires an existing entry, decrypts it, replaces only the
+bytes before its first newline, and preserves that newline and every following
+byte exactly. The generated password is returned separately as redacted,
+zeroizing data for typed terminal, clipboard, or QR presentation.
+
+The completed entry is encrypted for the nearest recipient policy and written
+atomically. Embedded Git receives `Add generated password for ...` only after a
+successful write. Decline, missing in-place targets, validation, randomness,
+encryption, and commit failures leave the previous repository state intact;
+commit failure uses the write-domain rollback path.