Allow First Mate to target already-ready units

[abueide]

Problem

First Mate says to ready another unit. The implementation adds an unprinted Exhausted targeting restriction, excluding legal already-ready targets and sometimes auto-selecting the only exhausted unit instead of offering the intended choice.

Change

• Retain Unit and NotSelf, remove only Exhausted.
• Cover ready/exhausted other units of either controller, self exclusion, ready-target no-op, normal readying, and no-other-unit resolution.
• No wire, blob, or snapshot changes.

Card reference: https://playriftbound.com/en-us/card-gallery/#card-gallery--ogn-132-298

Validation

• Focused First Mate tests: 4 passed per repository.
• Full turns library: Agni 4,580 passed / 172 ignored; agni-rfb 4,566 passed / 172 ignored.
• bash ci/check.sh and changed-file rustfmt check passed in both repositories.
• Direct treefmt could not initialize because taplo/alejandra were missing from PATH. Wasm linking was blocked by missing lld. These are not represented as successful builds.
• No live Kai/GUI or network playtest claimed.

Integration

The implementation is mirrored because Kai still consumes the Agni copy. A companion repository PR is linked separately. Kai must consume the changed dependency/module before players receive the fix.

[abueide]

Mirrored rules PR: abysl/agni-rfb#2 . Kai integration will pin the combined audit fixes; no main branch has been merged by this task.