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/01-syntax-design.md

@@ -0,0 +1,68 @@
+# 01 — Design and finalize syntax for all new features
+
+**Status:** resolved
+**Blocks:** #02, #04, #05
+
+## Backtick command execution
+
+Backtick expressions return a plain table with three fields:
+
+```lua
+local output = `ls -lha`
+print(
+  output.code,
+  output.stdout,
+  output.stderr
+)
+```
+
+- `output.stdout` — captured stdout as a string
+- `output.stderr` — captured stderr as a string
+- `output.code` — integer exit code
+- No `__tostring` metamethod — access `.stdout` explicitly
+
+### String interpolation
+
+Backticks support `${expr}` interpolation:
+
+```lua
+local dir = "/tmp"
+local result = `ls -lha ${dir}`
+local pattern = "*.lua"
+local result2 = `find ${dir} -name ${pattern}`
+```
+
+`${expr}` evaluates the Lua expression, converts to string, and splices it into the command string before argv parsing.
+
+## Environment variables — `$` sigil
+
+```lua
+local dir = $PWD              -- getenv("PWD") → string or nil
+$PWD = "/"                    -- setenv("PWD", "/")
+$MY_ENV_VAR = "data"          -- setenv("MY_ENV_VAR", "data")
+```
+
+- `$NAME` as an expression → `getenv("NAME")`, returns string or `nil`
+- `$NAME = expr` as a statement → `setenv("NAME", tostring(expr))`
+- `$` followed by an identifier is a new token type in the lexer
+- Note: `$NAME` (env var) is distinct from `${expr}` inside backticks (interpolation)
+
+## Piping (future)
+
+```lua
+local result = `ls -l` | `grep ".lua"`
+```
+
+Challenge: `|` conflicts with Lua's bitwise OR. Pipes only valid between command expressions.
+
+## Redirection (future)
+
+```lua
+`ls -l` > "output.txt"
+`ls -l` >> "output.txt"
+`cmd` 2> "err.txt"
+`cmd` 2>&1
+`cmd` < "input.txt"
+```
+
+Challenge: `>` / `>>` conflict with Lua's comparison/shift operators. Only valid in command context.