[FEATURE] Native RTL (Right-to-Left) alignment in TUI renderer #39525

Description

• I have verified this feature I'm about to request hasn't been suggested before.

The CLI/TUI renderer currently assumes all text is Left-to-Right (LTR). When a user configures OpenCode to speak an RTL language (Arabic, Persian, Hebrew) or uses an RTL plugin like opencode-rtl, the text's unicode character ordering can be fixed via BiDi control characters, but the text remains strictly left-aligned.

In right-to-left languages, reading left-aligned text is incredibly jarring. We need a way for the TUI to right-align blocks of text that are predominantly RTL.

Why this is not a duplicate:

• Support RTL/BiDi text rendering in CLI output #38524 reports BiDi character ordering bugs — this issue is specifically about paragraph alignment (right-aligning text in the terminal), which is a different rendering concern.
• [FEATURE]: Add RTL (Right-to-Left) language support in the chat panel #26915 targets the Desktop/Electron app (CSS dir attribute) — this issue targets the CLI/TUI renderer (terminal padding/alignment).
• Persian/Arabic (RTL) text is not displayed correctly in opencode TUI #32984 reports that Persian/Arabic text is "not displayed correctly" — this issue proposes a concrete implementation for right-alignment specifically, which none of the above issues address with a proposed solution for the TUI.

Proposed Solution

Add native support to the TUI renderer to detect and align RTL text.

1. Detection: When rendering a block of text (paragraph, list item, etc.), check if the first strong directional character is RTL (or if the block contains a high percentage of RTL characters).
2. Alignment: If the block is RTL, pad the text with spaces on the left side instead of the right side, effectively right-aligning it in the terminal window.
3. Configuration: This could be an explicit setting in opencode.json (e.g., "ui": { "rtl": true }) or auto-detected.

Alternative Solution

If native detection is too complex for the TUI renderer (Ink/React), expose a styling primitive or terminal width variable to plugins so they can pad strings manually to achieve right-alignment.

Context

See the opencode-rtl community plugin (https://github.com/razavioo/opencode-rtl) which solves the unicode bidi ordering problems, but is currently blocked from fully solving the UX because it cannot control terminal alignment.