feat(delegation): uncap max_spawn_depth (floor 1, no ceiling) (#39772)

* fix: respect disabled auto-compaction on context overflow

Port from anomalyco/opencode#30749.

When compression.enabled is false, NO automatic compaction trigger may
fire. The proactive token-threshold paths (preflight + post-response
should_compress gate) already honoured the setting, but the three
provider-overflow recovery paths in the agent loop — long-context-tier
429, 413 payload-too-large, and context-overflow — called
_compress_context() unconditionally, silently compressing and rotating
the session against the user's explicit choice.

Add a single guard at the top of the overflow-recovery dispatch: when
compression is disabled and the error is one of those three overflow
classes, surface a terminal error (compaction_disabled: True) telling the
user to /compress manually, /new, switch to a larger-context model, or
reduce attachments. Manual /compress (force=True) is unaffected — it never
enters this loop.

Tests: new TestOverflowWithCompactionDisabled (413 + 400 overflow don't
compress when disabled; control case still compresses when enabled).
Existing overflow-recovery tests updated to enable compaction explicitly
(they verify the recovery fires); fixture defaults flipped to True to
match production (compression.enabled defaults to True).

* feat(delegation): uncap max_spawn_depth to match max_concurrent_children

Removed the hard ceiling of 3 on delegation.max_spawn_depth. Depth now has
a floor of 1 and no upper limit, mirroring max_concurrent_children. Cost
(each level multiplies API spend) is the practical limiter, not a constant.

- delegate_tool.py: drop _MAX_SPAWN_DEPTH_CAP, _get_max_spawn_depth() floors
  at 1 instead of clamping to [1,3]; depth-limit error string reworded
- config.py / cli-config.yaml.example: doc comments say floor 1, no ceiling
- docs (configuration, delegation, delegation-patterns): range 1-3 -> >=1
- tests: convert clamp-above-3 change-detector into a no-ceiling invariant,
  drop the _MAX_SPAWN_DEPTH_CAP==3 snapshot assert, fix warning-text assert

cli-config.yaml.example

@@ -871,7 +871,7 @@ delegation:
   max_iterations: 50                          # Max tool-calling turns per child (default: 50)
   # max_concurrent_children: 3                # Max parallel child agents per batch (default: 3, floor: 1, no ceiling).
                                               # WARNING: values above 10 multiply API cost linearly.
-  # max_spawn_depth: 1                        # Delegation tree depth cap (range: 1-3, default: 1 = flat).
+  # max_spawn_depth: 1                        # Delegation tree depth (floor 1, no ceiling; default: 1 = flat).
                                               # Raise to 2 to allow workers to spawn their own subagents.
                                               # Requires role="orchestrator" on intermediate agents.
   # orchestrator_enabled: true                # Kill switch for role="orchestrator" children (default: true).