feat(attribution): pseudonymous file attribution (ESA-style)

Adapt Princess_Pi's Encrypt-Share-Attribution scheme so shared files are
anonymous but provably attributable.

A. In-session: a persistent Ed25519 persona key (~/.config/hack-house/
   persona_ed25519) signs every /send and /sendroom offer over the content
   hash; receivers verify it and see the persona fingerprint. Optional
   --attest <passphrase> attaches a revealable SHA-512(pass||sha256)
   commitment. Additive JSON — wire-compatible with the Python client.

B. Portable: /export-signed <dir> packages a directory into Princess_Pi's
   exact ESA 7z (fresh per-round Ed25519 sig over an inner 7z, SHA-512
   checksums, bundled verify scripts). Builder embedded from
   hh/tools/esa/esa_build.sh; verifiable with just bash+7z+ssh-keygen.

Tests: 45 pass (3 new persona). ESA archive build + verify-everything.sh +
passphrase reveal verified end-to-end.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/attribution.md

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+# Pseudonymous attribution
+
+hack-house lets you share files **pseudonymously but provably** — a recipient can
+verify *who* authored a file (and that it's intact) without anyone, including the
+relay server, learning your real identity. It adapts Princess_Pi's
+**Encrypt-Share-Attribution** scheme from the Church of Malware codex
+(<https://git.thecoven.info/PrincessPi/Encrypt-Share-Attribution>).
+
+There are two independent ways to prove authorship, mirroring ESA:
+
+1. **Persona signature (automatic).** On first run each client mints a long-lived
+   Ed25519 "persona" key at `~/.config/hack-house/persona_ed25519` (0600). Every
+   `/send` / `/sendroom` offer carries the persona public key and a detached
+   signature over `attest-v1 || sha256 || name || size`. Receivers verify it and
+   see the persona **fingerprint** (`⛧<8 hex>`), so the same author is recognizable
+   across offers. The fields are additive JSON — a Python peer that doesn't sign
+   still interoperates (its offers just show as *unsigned*).
+
+2. **Attribution passphrase (opt-in).** Add `--attest <passphrase>` to a send:
+   the offer then carries a commitment `SHA-512(passphrase || sha256)`. You can
+   *later* reveal the passphrase to prove authorship to anyone, even people who
+   weren't in the room.
+
+## Commands
+
+```
+/send <user> <path> [--attest <passphrase>]
+/sendroom <path>     [--attest <passphrase>]
+/export-signed <dir> [--attest <passphrase>]
+```
+
+`/export-signed` packages a directory into a **portable, self-verifying ESA 7z
+archive** (`verifiable_archive_<ts>.7z`) using Princess_Pi's exact format: a fresh
+per-round Ed25519 key signs an inner `contents.7z`, SHA-512 checksums cover the
+outer layer, and bundled `verify-everything.sh` / `test_validate_passphrase.sh`
+let anyone with `bash` + `7z` + `ssh-keygen` verify it — no hack-house needed. The
+builder is `hh/tools/esa/esa_build.sh` (embedded in the binary). Requires `7z`,
+`ssh-keygen`, `sha512sum`, `shred`, `openssl` on the host.