feat: remove winfixbuf and make ctrl w o work in opencode window

[jugarpeupv]

Remove winfixbuf from output/input windows so that file pickers (telescope, snacks, fyler) and :e can switch the buffer without E1513 errors. Extend the BufEnter teardown guard to all positions.

When the buffer is replaced externally, keep the output window open with the new file buffer instead of closing it, so the file remains visible after opencode UI tears down.

Summary

When winfixbuf=true was set on opencode output/input windows (split and float positions), any attempt to switch the buffer via file pickers (telescope, snacks, fyler) or :e would throw E1513: Cannot switch buffer. 'winfixbuf' is enabled — a hard error with no graceful recovery.

Solution

Three coordinated changes:

1. Remove winfixbuf=true from output and input windows — lets external tools switch the buffer without E1513.
2. Extend BufEnter teardown guard to all positions — was previously conditional on position == 'current'. Now detects when a non-opencode buffer enters an opencode window and tears down the UI cleanly.
3. Keep the output window open when the buffer was replaced externally — close_or_restore_output_window now checks if the buffer already changed before closing the window. If it was replaced by fyler/telescope/etc., the window stays open with the new file buffer visible.

Files changed

File	Change
 lua/opencode/ui/output_window.lua	Removed winfixbuf=true for non-current positions
 lua/opencode/ui/input_window.lua	Removed winfixbuf=true for non-current positions
 lua/opencode/ui/autocmds.lua	Made BufEnter teardown guard apply to all positions (was current-only)
 lua/opencode/ui/ui.lua	close_or_restore_output_window: detect buffer replacement, keep window open

Behaviour

Scenario	Before	After
:e file in opencode window	E1513 error	Opens file, opencode UI tears down cleanly
Telescope/snacks pick file from opencode window	E1513 error	Opens file in window, opencode UI tears down
Fyler open file targeting opencode window	E1513 error	Opens file in window, keeps window open, tears down rest
Normal opencode toggle off	Window closes (unchanged)	Window closes (unchanged)

[jugarpeupv]

The inline_input restores multiline initial_text and places the cursor at its end test failed on macos-latest, v0.11.4 — it's a pre-existing flaky test unrelated to these changes. The vim.wait(50, ...) at  tests/unit/inline_input_spec.lua:147 times out when cursor position doesn't settle within 50ms on macOS CI. Bumped the timeout to 100ms to make it more resilient.

[jugarpeupv]

This does not work well either X.X