screenDiff returns wrong content for AgentTypeOpencode

[mafredri]

screenDiff (formerly FindNewMessage) incorrectly includes header lines and old content in the diff when used with AgentTypeOpencode.

Observed behavior:
When extracting new messages from Opencode agents, the diff includes:

• Dynamic header lines with token counts/costs (┃ /share ... 15.3K/8% ($0.07) ┃)
• Old content that appeared in the previous screen
• Only partial new content

Expected behavior:
Should return only the new message content, excluding header metadata and previously-seen lines.

Example:

oldScreen := `┃ Header 12.6K/6% ($0.05) ┃
old line A
old line B`

newScreen := `┃ Header 15.3K/8% ($0.07) ┃
old line A
old line B
new message`

// Currently returns: "┃ Header 15.3K/8% ($0.07) ┃\nold line A\nold line B\nnew message"
// Should return: "new message"

Impact:
Opencode users see garbled responses with header metadata mixed into agent messages.

Probable cause:
The logic that skips the first two header lines (when dynamicHeaderEnd=2) uses a sliced array in the loop. The index variable is relative to the slice, not the original array, causing an off-by-one error.

Test coverage gap:
All existing screenDiff tests use AgentTypeCustom, which doesn't set dynamicHeaderEnd, so the bug path is untested.