feat(builtins): add dotenv command (non-standard, .env file loader)

[chaliy]

Summary

A non-standard builtin to load .env files into the shell environment. Every modern project has a .env file. Currently requires export $(cat .env | xargs) which breaks on quotes, comments, and multiline values.

Proposed Syntax

dotenv [OPTIONS] [file...]

Proposed Flags

Flag	Description
(default)	Load .env in current directory
-f FILE	Load specific file
-e / --export	Also export variables (default: shell-local)
-o / --override	Override existing variables (default: skip existing)
-p / --print	Print loaded variables without setting them
--prefix PREFIX	Add prefix to all variable names

.env Format Support

# Comments
KEY=value
KEY="quoted value"
KEY='single quoted'
KEY="multi\nline"
EMPTY=
# No value (set to empty)
BARE_KEY

# Variable interpolation
BASE=/app
DATA=${BASE}/data

Use Cases

# Load defaults
dotenv

# Load specific env
dotenv -f .env.production --export

# Load without overriding existing
dotenv -f .env.defaults

# Preview what would be loaded
dotenv -p -f .env.staging

# Load with prefix (for namespacing)
dotenv --prefix DB_ -f database.env
# DB_HOST, DB_PORT, etc.

llm_hint()

Some("dotenv: Load .env files. `dotenv` loads .env, `dotenv -f .env.prod -e` loads and exports. Handles quotes, comments, interpolation.")

[chaliy]

Documentation Requirements

Since this is a non-standard (invented) builtin, it must be documented in two places:

1. README.md — Add to the "Built-in Commands" table under a new Extensions category row (non-standard bashkit-specific commands, clearly marked as extensions)
2. crates/bashkit/docs/extensions.md — Create or update a dedicated extensions guide covering all invented builtins, with short descriptions, examples, and rationale for why each exists

This ensures users discover these commands and understand they are bashkit-specific, not standard Bash.

[chaliy]

Resolved in PR #585 or #588