feat(ui): run post-flush command without an output file

[umputun]

O now runs --post-flush-command when no output file is set, so a single config line gives an in-session copy of the annotations:

post-flush-command = pbcopy

that works in every session, including a bare revdiff HEAD~1, with no per-launch flags and no output file.

the -o requirement was never a design decision. #277 added the hook coupled to --output, and its author said in that thread that running it standalone made sense as a follow-up but would have grown the diff. Nothing validated the coupling, it was just the early return at the top of handleFlushOutput.

behavior now, by what is configured:

• output file only: unchanged, writes the file, same hints
• command only: formats the snapshot, writes no file, sends it to the command on stdin
• both: writes the file, then sends the exact string WriteFile returned, so the file and the command cannot drift
• neither: status hint naming both options, nothing happens

an empty store is checked before any of that, so O with no annotations still says so rather than running a command with nothing to send. Tests cover all four modes plus hook-only success and failure.

docs updated in step: the flag description no longer claims it requires -o, and README, site/docs.html, the Claude and Codex reference pages, the Pi skill note, docs/ARCHITECTURE.md and the gotchas entry all say what the flag actually does now. README also gains the pbcopy recipe, which was the thing #336 could not find, with xclip -selection clipboard for X11 and wl-copy for Wayland, and OSC 52 kept as the answer for ssh and multiplexers.

docs/backlog/post-flush-clipboard-route-undocumented.md is deleted here, since this is the change that documents the route.

Related to #336. Follow-up to #277.

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