Implement shell package with readline vi mode and tab completion (task 358)

Replaces the stub in internal/shell/shell.go with a full implementation
backed by github.com/ergochat/readline.  The Shell struct wraps a readline
instance configured with vi mode, a 500-entry in-memory history limit, and
a custom prefixCompleter that delegates tab expansion to a caller-supplied
function — mirroring the Ruby CLI#setup_readline / Readline.completion_proc
pattern.  History deduplication (skip empty lines and consecutive duplicate
entries) matches the Ruby shell_loop behaviour by disabling auto-save and
calling SaveToHistory only for non-empty, non-duplicate lines.  A package-
level ReadPassword helper handles PIN entry before the Shell is created.
Tests skip gracefully when stdin is not a TTY.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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