Add pipe-friendly plain-text defaults to list commands

[polymorcodeus]

Summary

Adds plain-text default output to non-interactive list commands so book composes with fzf, jq, and shell pipelines without requiring --format. Structured JSON/TOML output remains available via --format.

What's new

• Plain-text defaults - shelf list, collection list, mark list, and mark list --trash now print one item per line when --format is omitted.
• Removed format enforcement - the Before hooks that hard-errored on missing --format are gone; --format is now opt-in structured output.
• Consistent mark formatting - normal and trashed mark lists both default to title URL per line, matching mark search.
• Documentation updates - README and AGENTS.md now describe the plain-text default and the optional --format flag.

Usage

Command	Purpose
book shelf list	List shelf names, one per line
book collection list --shelf <name>	List collection names, one per line
book mark list --shelf <name> --collection <name>	List marks as title URL
book mark list --trash	List trashed marks as title URL
book <list command> --format json	Opt into JSON output
book <list command> --format toml	Opt into TOML output

Behavior changes / guardrails

• Interactive mode is unchanged; --format only affects non-interactive list output.
• Empty --format now yields plain text instead of an error.
• The existing JSON/TOML output format and content are unchanged.

Testing

• Smoke-tested plain-text defaults and structured JSON/TOML output for shelf list, collection list, and mark list.
• make check runs fmt, vet, lint, and tests; all green.