Implement shell builtins: cd, exec, umask (issue #15)

Add builtin dispatch in lcmd.c that checks __builtins table before
fork(), so cd/exec/umask operate on the shell process itself.

issues/15-builtins.md

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+# 15 — Shell builtins
+
+**Status:** done
+
+Some commands must run inside the shell process itself to have any effect. An external `cd` binary exists on most systems but it spawns a subprocess, changes directory there, and exits — leaving lush's own working directory unchanged.
+
+Lush only needs builtins for operations that **cannot be implemented in pure Lua or via external binaries** — things that require modifying the shell process state via syscalls with no Lua API.
+
+## Assessment
+
+Every builtin from `man 1 builtin` was evaluated against two questions:
+1. Can it be done in Lua? (e.g. `export` → `$VAR = "val"`, `source` → `dofile()`, `exit` → `os.exit()`)
+2. Does the external binary work? (e.g. `/bin/pwd`, `/usr/bin/which`, `/usr/bin/kill`)
+
+If either answer is yes, it's not needed as a builtin. What remains:
+
+| Builtin | Why it must be a builtin |
+|---------|-------------------------|
+| **`cd`** | `chdir(2)` must happen in the lush process. No Lua API. |
+| **`exec`** | `execvp(2)` replaces the current process. `os.execute()` spawns a subprocess. No Lua equivalent. |
+| **`umask`** | `umask(2)` is per-process. The external `/usr/bin/umask` is a shell script that calls the shell's own builtin. No Lua API. |
+
+## Builtins
+
+### `cd [dir]`
+
+Change working directory via `chdir(2)`.
+
+- No argument: `cd $HOME`
+- `cd -`: change to `$OLDPWD`
+- Updates `$PWD` and `$OLDPWD` environment variables
+- Errors if directory doesn't exist or isn't accessible
+
+### `exec command [args...]`
+
+Replace the shell process with the given command via `execvp(2)`. Does not return on success. On failure, prints an error and the shell continues.
+
+### `umask [mode]`
+
+Get or set the file creation mask via `umask(2)`.
+
+- No argument: print current umask (octal)
+- With argument: set umask to the given octal value
+
+## Design considerations
+
+- **Dispatch order:** When a command is entered (backtick or `!` prefix), check builtins *before* searching `$PATH`. A builtin named `cd` must intercept the command before it reaches `execvp`.
+- **Implementation:** Expose the syscalls to Lua (e.g. `os.chdir()`, `os.exec()`, `os.umask()`), then implement the builtins as Lua functions registered at startup. This keeps the C side minimal and lets users compose with the primitives directly.
+- **Interaction with backticks:** `` `cd /tmp` `` and `!cd /tmp` should both trigger the builtin, not spawn an external process.
+
+## Open questions
+
+- Should builtins be overridable by the user?
+- Should there be a `builtin` command to bypass user overrides?