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/06-piping.md

@@ -0,0 +1,25 @@
+# 06 — Implement piping between commands
+
+**Status:** open (post-core)
+**Blocked by:** #03, #04
+
+## Syntax
+
+```lua
+local result = `ls -l` | `grep ".lua"`
+```
+
+## Implementation
+
+- Connect stdout of one child to stdin of the next via `pipe()` + `dup2()`
+- Build a pipeline of N processes, all forked and connected
+- Only capture stdout/stderr of the **final** process
+- `waitpid()` all children, return exit code of last process
+
+## Challenge
+
+Lua uses `|` for bitwise OR. The parser needs to disambiguate:
+- `|` between two command expressions → pipe
+- `|` between numeric expressions → bitwise OR
+
+This is resolvable because the parser knows the type of each subexpression — a backtick expression is always a command.