Follow-up to the salvaged #35533 fix: instead of parsing 'here [N]' only in
_mirror_slash_side_effects, parse it inside _compress_session_history — the
single helper all three manual-compress routes converge on (session.compress
RPC, command.dispatch /compress|/compact, slash-exec mirror). Every route now
honors the boundary-aware forms (here [N], up to here, --keep N) with the
same head/tail split + rejoin as cli.py and gateway/slash_commands.py, and
keeps the choke point's existing guards (lock-free LLM call, history_version
race check, deferred context-engine notification).
Tests: choke-point unit tests for 'here N', degenerate-split fallback, and
focus-topic passthrough, plus endpoint-level tests on each of the three
routes.
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