feat(cli): Unify turn selection across print/compact/fork

[JeanMertz]

jp c fork now accepts the same turn selector as jp c print and jp c compact: --from/--to, --turn, --first/--last, and --keep-first/--keep-last, replacing its old bespoke --from/--until/--first/--last flags. --from/--to also gain duration, absolute date (2026-01-01), and RFC 3339 support, and every bound now resolves to a whole turn instead of splitting one: --from <time> starts at the first turn to begin after the cutoff, --to <time> ends at (and includes) the turn active at the cutoff.

--first N --last M given together now compact/fork/print each window separately and skip the turns between them, rather than the previous single-range behavior. --keep-first/--keep-last compose with every selector (not just --first/--last) to protect turns at either end, and an explicit --keep-first/--keep-last replaces a configured compaction rule's bound instead of stacking with it.

The conversation-creation range filter on jp c rm, jp c archive, and jp c use is renamed from --from/--until to --created-since/--created-before, freeing --from/--to to mean "which turns" everywhere and --created-since/--created-before to mean "which conversations" — the two families accept near-identical values, so the old shared names were ambiguous. Semantics of the creation-range filter (half-open, since inclusive/before exclusive) are unchanged.

crates/jp_cli/src/cmd/turn_range.rs is replaced by turn_selection.rs, which resolves a selection to an ordered, non-overlapping TurnSet of inclusive 0-based windows instead of a single from/to pair, and ConversationStream::retain_first_turns/ retain_first_and_last_turns are replaced by a single retain_turns(impl Fn(usize) -> bool).