[feat]: Add contextual actions to user messages

[jensenojs]

Summary

• expose Revert, Copy, and Fork through the existing contextual actions UI
• keep one action set across the full rendered user message range
• restore overwritten buffer-local mappings when contextual actions become inactive
• copy original non-synthetic user text through the existing API command axis

Model

state.messages                         RenderState
├── message id ──> undo / fork         └── user message range
└── original text ──> copy                 └── R / C / F

cursor line ──> actions in range ──> temporary mappings ──> opencode.api
                       │
                       └── leave range ──> restore previous mappings

Testing

• ./run_tests.sh
• 138 replay cases
• contextual action mapping, persistent-buffer, range-shift, and Copy contract coverage

Closes the interaction gap discussed in #439 by reusing contextual actions instead of adding another dialog or mouse-specific surface.

[jensenojs]

test for copy

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test for revert

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I also tested the fork(but I'm too lazy to record another GIF); this feature might make you more inclined to try adjusting your prompts by reverting the previous message @sudo-tee

[sudo-tee]

This is great.

I much prefer this version with contextual actions on the user message than the previous one :)

Thanks