feat(newos4): context window warns at 80% and says what trimming dropped (v4.6.91)

P0-5 asked what to do about a 422k-token session against a 400k threshold. There is no 400k
threshold in this tree — the budgets are maxContextTokens (12000) and maxContext*2 messages — so
rather than chase a number that does not exist, this applies the decision that is defensible
either way: warn before the cliff, never lose history without saying so.

- 80% of the token budget prints one warning per crossing (no per-turn nagging)
- the message-count trim unshifts a marker naming how many messages left the window and quoting
  the earliest one, instead of splicing them away silently
- 'max-context-tokens (default: 2000)' now reports the real code default, 12000

Verified: 900/1000 tokens -> warns once, second call silent; 1500/1000 -> compresses to 17
messages with the summary head intact; 9-message window with maxContext=2 -> 4 messages, marker
'[context trim: 6 earlier message(s) ... earliest was: "turn 0 ..."]'.

0 files changed, 0 insertions, 0 deletions