[v0.3] Add multiple buffers, find-file, and switch-buffer

[owainlewis]

Goal: Add a small buffer list so Cortex can open, create, and switch between multiple files in one session.

Context: Splits and tabs are much more useful once the editor has real buffers underneath them.
The current editor has one active buffer and a directory picker at startup or /open time.
Cortex can already open a missing path as an empty buffer from the CLI or /open, but C-x C-f only exposes a picker of existing files.
Emacs-style find-file must also accept a filename that does not exist yet and create it when the buffer is saved.
This issue should introduce the buffer model without changing the app into a project explorer.

Proposed approach:

• Add an editor-level buffer list with one active buffer id or index.
• Preserve per-buffer dirty state and file path.
• Add Emacs-style find-file behavior through C-x C-f.
• Let the find-file prompt accept an editable path, including a filename that does not exist.
• Open a missing path as an empty buffer associated with that path; create the file only when the buffer is saved.
• Keep the existing rule that saving does not create missing parent directories.
• Add switch-buffer behavior for choosing an already-open buffer.
• Reuse the minibuffer or existing picker where practical.
• Keep each buffer's view state either in the buffer entry or a small associated view map, with splits in mind.
• Preserve current startup behavior for opening a file or directory.

Acceptance criteria:

• Opening a second file keeps the first buffer in memory.
• Switching buffers changes the visible file without losing unsaved edits.
• Dirty state is tracked per buffer.
• Saving saves the active buffer only.
• Quit warns if any open buffer is dirty, or has a clear scoped behavior documented in the issue plan.
• C-x C-f opens an editable find-file prompt that accepts a file path.
• Entering an existing file path opens that file.
• Entering a missing file path opens an empty buffer associated with that exact path.
• Saving that buffer creates the file when its parent directory exists.
• Saving reports a clear error and keeps the buffer open when the parent directory does not exist.
• Canceling find-file leaves the active buffer unchanged.
• C-x b switches between open buffers.
• Existing directory browsing and single-file editing behavior still works.

Verification:

• Run cargo test.
• Manually open two existing files, edit both, switch between them, save one, save the other, and quit.
• Manually use C-x C-f to enter a missing filename, add text, save it, and confirm the file was created with the expected contents.
• Manually use C-x C-f with a path whose parent directory is missing and confirm save reports a clear error without losing the buffer.
• Manually cancel find-file and confirm the active buffer is unchanged.
• Manually confirm dirty quit behavior with one dirty inactive buffer.
• Manually confirm terminal cleanup after exit.

Out of scope:

• Splits and tabs.
• Project tree/sidebar.
• Recursive fuzzy file search.
• Recent files.
• Creating missing parent directories.
• External file reload.

[owainlewis]

Implemented in ready PR #57: #57

Proof:

• C-x C-f accepts existing, missing, exact whitespace-bearing, and directory paths.
• Missing paths open as empty buffers and are created on save when the parent exists.
• Multiple buffers retain independent unsaved text, dirty state, and view state.
• C-x b switches by exact path or unique file name.
• Quit protects dirty inactive buffers.
• Directory picker browsing remains available mid-session.
• cargo fmt --check, strict Clippy, all 155 tests, and git diff --check pass.
• Two real terminal smoke tests passed with clean shell restoration.
• Fresh-agent re-review returned no findings.