* fix: add claude-opus-5 to KNOWN_MODEL_WINDOW_TOKENS
claude-opus-5 has a 1M context window (verified: 250k tokens at 25%
usage = ~1M), but was missing from the model table. This caused
resolveContextWindowTokens() to fall back to the 200k default when
tokens < 200k, incorrectly triggering compact warnings in the first
20% of a 1M session.
Same failure class as #2290 (Opus 4.x) and #2461 (fable-5/mythos-5).
The env override (ECC_CONTEXT_WINDOW_TOKENS) remains the escape hatch
for unlisted models.
Refs: #2290, #2461, #2468
* test: add regression test for claude-opus-5 context window
Verifies resolveContextWindowTokens returns LARGE_CONTEXT_WINDOW_TOKENS
for claude-opus-5 at 50k tokens, matching the behavior of fable-5 and
mythos-5 in the known-model table.
