Pinning: store stable IDs and add pin/unpin/replace/recompute UI actions

[multiplex55]

Motivation

• Pinned history entries should refer to stable action identifiers (and args) rather than transient display text so pins remain resolvable across data/label changes.
• Provide an easy way to pin the currently shown query result and to refresh pinned metadata when plugins/data change.
• Surface missing or removed pinned items in the UI and allow users to unpin or replace them.

Description

• Change pin storage and identity to use action_id + args and add helper methods on HistoryPin (matches_action, matches_id, update_from_action).
• Add persistent pin management APIs in src/history.rs: upsert_pin, remove_pin, and recompute_pins (returns PinRecomputeReport), and adjust PartialEq for HistoryPin to use the new identity.
• Extend the command history widget (src/dashboard/widgets/command_history.rs) to resolve pinned entries against current actions/plugins/data, mark missing pins, and expose an Unpin affordance for missing entries.
• Add result-context pin actions in the main UI (src/gui/mod.rs): Pin current query result, Unpin result, Replace pin with current result, and Recompute pinned results, plus resolve_pin_action and pin_result_menu helpers integrated into existing result context menus.
• Add unit tests around pin behavior in src/history.rs (see test names below).

Testing

• Added unit tests in src/history.rs: pin_roundtrip_and_toggle (existing), pin_identity_uses_action_id_and_args, and upsert_and_recompute_pins to validate identity, upsert, recompute and remove semantics.
• No automated test suite was executed in this rollout (tests were added but not run locally as part of this change).

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Codex Task