Add project issues for lush (Lua + shell) features

Defines syntax decisions and implementation plan across 7 issues:
- backtick command execution returning {code, stdout, stderr}
- ${expr} interpolation in backticks
- $SIGIL env var read/write
- argv parsing, piping, and redirection (future)

issues/03-command-execution-runtime.md

@@ -0,0 +1,42 @@
+# 03 — Implement direct command execution runtime (no shell)
+
+**Status:** open
+**Blocked by:** #02
+**Blocks:** #06, #07
+
+Implement the C runtime that executes commands when a backtick expression is evaluated.
+
+**Critical constraint:** do NOT spawn a shell (no `bash`, `dash`, `sh`, `system()`, `popen()`).
+
+## Return value
+
+Backtick expressions return a table:
+
+```lua
+local r = `ls -lha`
+r.code    -- exit code (integer)
+r.stdout  -- captured stdout (string)
+r.stderr  -- captured stderr (string)
+```
+
+## Implementation approach
+
+- `fork()` + `execvp()` on Unix/macOS
+- Set up two pipes: one for stdout, one for stderr
+- In child: `dup2()` pipes onto fd 1 and fd 2, then `execvp(argv[0], argv)`
+- In parent: read both pipes into buffers, `waitpid()` for exit status
+- Build a Lua table with `code`, `stdout`, `stderr` fields and push onto stack
+- Depends on #04 (argv parsing) to tokenize the command string into argv[]
+
+## Error handling
+
+- Command not found → set `code` to an error value, `stderr` to error message
+- Permission denied → same
+- `fork()` failure → raise a Lua error
+
+## Open questions
+
+- New file (`lcmd.c` / `lcmd.h`) or add to an existing lib?
+- Signal handling during child execution
+- Binary output — return raw string bytes as-is?
+- Set `__tostring` metamethod on result table?