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/05-env-var-access.md

@@ -0,0 +1,37 @@
+# 05 — Implement environment variable access (read/write)
+
+**Status:** open
+**Blocked by:** #01
+
+Add syntax for reading and writing environment variables directly from Lua.
+
+## Option 1 — `export` keyword
+
+```lua
+local var = "my data"
+export var                    -- exports existing local to env
+export other_var = "my data"  -- creates + exports
+```
+
+- New reserved word `export` in lexer
+- `export var` looks up the local's value by name and calls `setenv()`
+- `export x = val` combines declaration with `setenv()`
+
+## Option 2 — `$` sigil
+
+```lua
+local dir = $PWD              -- getenv("PWD")
+$PWD = "/"                    -- setenv("PWD", "/")
+$MY_ENV_VAR = "data"          -- setenv("MY_ENV_VAR", "data")
+```
+
+- `$NAME` as expression → `getenv("NAME")`, returns string or `nil`
+- `$NAME = expr` as statement → `setenv("NAME", tostring(expr))`
+- New token type in lexer for `$` + identifier
+
+## Implementation touches
+
+- `llex.c` / `llex.h` — new token(s) or reserved word
+- `lparser.c` — new expression/statement rules
+- Runtime calls `getenv()` / `setenv()` — standard C, no shell needed
+- Non-existent env var → return `nil`