phase0 amendment: vendor dkjson 2.8 under vendor/

Captures the JSON-library decision noted as open in CLAUDE.md §6.
dkjson is pure Lua (preserves §3's "no compiled extensions" invariant),
single file, redistributable (MIT/X11). Sourced from Debian's `lua-dkjson`
package (/usr/share/lua/5.1/dkjson.lua, version 2.8) — Debian's curated
copy of the upstream at dkolf.de.

Vendoring (rather than relying on a system lua-dkjson install) keeps
aish self-contained per the §3 "no luarocks packages" invariant: any
host with luajit can run the tree as-is.

PHASE0.md §3 grows one row recording the choice.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/PHASE0.md

@@ -41,6 +41,7 @@ Phase 0 is the minimal working skeleton. It establishes the REPL loop, input dis
 | Shell execution | `io.popen` in Phase 0; `forkpty` via libc FFI from Phase 1 | `popen` sufficient for non-interactive commands; PTY required for vim, htop, etc. |
 | Session persistence | Deferred to Phase 1 | Phase 0 holds history in memory only |
 | Config format | Lua table (plain `.lua` file sourced at startup) | No parser dependency; native types; easily extended |
+| JSON encode/decode | dkjson 2.8 vendored under `vendor/dkjson.lua` | Pure Lua (preserves §3 "no compiled extensions" invariant); single-file vendor avoids `luarocks`; sourced from Debian's `lua-dkjson` package, originally from dkolf.de |
 
 ---