Add plain output to park show for pipes and redirects

[polymorcodeus]

Summary

Adds a plain-text rendering path to park show: output switches to glamour's notty style (no ANSI escapes) whenever stdout is not a terminal, and a new --plain flag forces the same behavior on a terminal. park assist applies the same rule to the note it opens after the TUI exits.

What's new

• Auto plain on non-TTY - park show detects a non-terminal writer and renders plain text, so piped and redirected output carries no ANSI escapes.
• --plain flag - forces plain output even on a terminal.
• Plain header - plain mode renders a category: ... created: ... source: ... header instead of the markdown-entity header used for rich output.
• Assist parity - the note park assist opens after the TUI exits follows the same non-terminal rule.

Usage

Command	Purpose
park show <file>	glamour render on a terminal, plain text when piped
park show <file> --plain	force plain text output

Behavior changes / guardrails

• Detection is writer-based: any writer that is not an *os.File (buffer, pipe) is treated as non-terminal, which keeps tests deterministic.
• No escape hatch to force rich output when piped; piping is exactly when plain is wanted.
• The glamour renderer cache now memoizes per style (dark and notty) under the same sharing model as before.

Testing

• internal/render tests cover rich, plain (no ANSI, no literal  ), and missing-file paths.
• cmd/park dispatch tests cover --plain and the auto non-TTY path.
• make check runs fmt, vet, lint, and tests; all green.