refactor(cli, conversation): Overhaul conversation targeting system

[JeanMertz]

To start, here is the new targeting help text, giving an overview of the targeting overhaul:

Conversation Targeting

Use a conversation ID (e.g. jp-c17761673600), a keyword, or any text to
fuzzy-search by title.

Interactive Filter/Picker:
  ?                             # select from all
  ?p, ?pinned                   # select from pinned
  ?s, ?session                  # select from session

Conversation Aliases:
  l, latest                     # target latest active in workspace
  n, newest                     # target newest created
  p, pinned                     # target latest pinned
  s, session                    # target previous active in session

Multi-Target Keywords:
  +p, +pinned                   # target all pinned
  +s, +session                  # target all activated in session

This information can be gotten either when using help as the target name (as a positional argument such as jp conversation use help, or using the --id=help flag), or --help on any command that takes a conversation target.

Additionally, pinned conversations now track their pin time, allowing cross-pinned conversations to be sorted by pin time. This sorting is applied both in jp conversation ls and interactive target pickers.

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As for the internals, here is what changed:

Replace the pinned: bool field on Conversation with pinned_at: Option<DateTime<Utc>>, recording when a conversation was pinned. This enables ordering pinned conversations by pin time rather than treating them all as equivalent. A new is_pinned() helper is provided for callers that only need the boolean.

The ConversationTarget enum is redesigned around clearer semantics:

• LastActivated → Latest (l, latest)
• LastCreated → Newest (n, newest)
• Previous → SessionPrevious (s, session)
• Current is removed
• LatestPinned added (p, pinned) — resolves the most recently pinned conversation
• Pinned added (+p, +pinned) — resolves all pinned conversations
• Session now uses +s/+session prefix to distinguish multi-target keywords from single-target aliases
• Help added to print keyword help and exit cleanly
• Interactive picker keywords are now prefixed with ? (?, ?p, ?pinned, ?s, ?session)
• Unknown input falls back to a fuzzy picker pre-seeded with the text, rather than returning a parse error

ConversationTarget::parse no longer returns Result; invalid conversation IDs now trigger the fuzzy picker instead of failing. Session-gating is moved to a new requires_session() predicate.

The ids fields on PositionalIds and FlagIds are made private behind a new ConversationIds trait exposing ids() and is_multi(). The ConversationLoadRequest builders now accept &dyn ConversationIds and propagate the multi flag, which switches the ? picker between single-select (inquire::Select) and multi-select (inquire::MultiSelect).

PickerFilter gains session and query fields. The query field pre-populates the picker's filter input, making any unrecognised text act as an instant fuzzy search.

Help text is rewritten to reflect the new keyword grammar and exposed via a standalone format_target_help() function used in error messages for NoConversationTarget and the new TargetHelp error.