Make the open_file integration test opt in

Opening a file pulls Obsidian to the foreground and takes keyboard focus
partway through a run, so keystrokes meant for another window land in
whatever note the test opened. It is the only test in the suite that
does this, and paying that cost on every run is a poor trade for one
assertion that the tool returns OK.

It now follows the pattern the active_file_* tests already use: skipped
unless its environment variable is set, here OBSIDIAN_TEST_OPEN_FILE=1.
The coverage is still there for anyone who wants it, it just no longer
interrupts whoever happens to be at the keyboard. Both opt-in variables
are now documented in AGENTS.md, which previously mentioned only
OBSIDIAN_API_KEY.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>