fix: support keybind overrides via plugin options

[bitIO]

Closes #5

Root cause

• tui.json keybinds accepts ONLY built-in keybind names; unknown keys like balance.toggle are silently dropped by the host (dropUnknownKeybinds). Config keybinds can never target plugin commands, so the old override path was inert and the plugin's api.tuiConfig.keybinds.has(...) skip-logic was dead code.
• The key parser treats uppercase as lowercase (uppercase != shift), so the default <leader>B was byte-identical to the built-in sidebar toggle <leader>b — the toggle actually toggled the sidebar.

Fix

• Keybind overrides now live in plugin options: "keybind": "ctrl+b" (toggle) and "refreshKeybind": "ctrl+r" (refresh); "none" disables a binding.
• Default toggle binding is now <leader>shift+b (truly distinct from the built-in sidebar toggle).
• Removed the dead api.tuiConfig.keybinds.has() conditional; extracted pure buildCommandBindings(opts) with unit tests.
• README updated (overrides via plugin options; tui.json keybinds documented as host-reserved).
• User's ~/.config/opencode/tui.json updated to the new channel.

Verification

• bun run typecheck, bun run build, bun test (47 pass) — all green locally; CI runs on this PR.