Add persistent terminal alias emulation

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Issue

Title

Make terminal aliases functional and persistent

Problem

The terminal command surface advertised alias and which, but alias only printed a static example and user-defined shortcuts could not be created, executed, or persisted. This left a common shell-emulation workflow missing from the portfolio terminal.

Acceptance Criteria

• Users can create aliases with alias name='command'.
• alias lists saved aliases and alias name inspects one alias.
• Aliases expand before command execution, preserve extra arguments, and work in supported pipe segments.
• which reports aliases before built-in commands.
• unalias name ... and unalias -a remove aliases.
• Aliases persist across page reloads in localStorage.

Changes

• Added tested alias parsing, formatting, and expansion helpers with loop detection.
• Wired persistent alias/unalias management into the browser terminal runtime, command completion, which, and pipeline execution.
• Documented alias usage in README and terminal help/man entries.

Verification

• pnpm check passed: lint, format check, 71 tests with coverage, and Astro build.
• Manual browser walkthrough recorded: created aliases, executed an alias, used one in a pipeline, reloaded to verify persistence, then removed aliases.