fix(compaction): default to off + raise unknown-model floor to 80% (#664)

Two coordinated changes that stop the engine from routinely rewriting the
prompt prefix and burning DeepSeek V4's prefix-cache discount:

1. `Settings::default().auto_compact` flips from `true` to `false`. The
   `auto_compact = on` opt-in and the explicit `/compact` slash command
   stay available for users / agents that decide their workload benefits
   from compaction more than from cache stability. With V4's 1M-token
   window the user has plenty of headroom to run long sessions without
   auto-trimming, and aggressive compaction has been the dominant
   cost-spike vector in long sessions (the rewritten prefix invalidates
   ~90% of the cache discount on every compaction event).

2. `DEFAULT_COMPACTION_TOKEN_THRESHOLD` raised from `50_000` to
   `102_400` (80% of `DEFAULT_CONTEXT_WINDOW_TOKENS = 128_000`). This is
   the last-resort threshold used when `context_window_for_model` returns
   `None` — i.e. an unrecognised model id. Pre-v0.8.11 the fallback
   compacted at ~5% of a V4 window when model detection silently fell
   through. Now the fallback inherits the same late-trigger discipline as
   the V4 path, so model-detection drift doesn't quietly burn cache.

Together: the two changes mean compaction never fires automatically by
default, and even when explicitly opted in (or when the runtime-thread /
capacity-flow paths invoke compaction with their own `enabled = true`
config), the threshold is anchored at 80% of the model's context window
(or 80% of the 128K default if the model is unknown), never below.

Tests
=====

- `default_settings_disable_auto_compact_to_protect_v4_prefix_cache` —
  pins the new default and explains the rationale inline.
- `auto_compact_remains_explicitly_configurable` — unchanged; still
  asserts the `set("auto_compact", "on" | "off")` round-trip works.
- `compaction_threshold_scales_with_context_window` — updated to assert
  `compaction_threshold_for_model("unknown-model") == 102_400`.
- `v4_soft_caps_only_apply_to_v4_models` — updated to assert the
  unknown-model + reasoning-effort path also lands on the new floor.

Verification
============

- `cargo fmt --all -- --check` clean.
- `cargo clippy -p deepseek-tui --bin deepseek-tui --all-features
  --locked -- -D warnings` clean.
- `cargo test -p deepseek-tui --bin deepseek-tui --locked` →
  2028 passed, 2 ignored.

Refs #664 (handoff-instead-of-compact pattern, full implementation
deferred). Behaviour-only change for v0.8.11; the larger
agent-aware-handoff mechanism is its own design surface.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -78,3 +78,6 @@ apps/
 # Local-only Claude / ralph notes
 .claude/*.local.md
 .claude/*.local.json
+
+# Maintainer-internal design notes (trade-secret material, never published)
+.private/