feat(mcp): M-level descent gating in read_context cascade + CoalescingBuffer fix

[jlunder00]

Summary

• M-level cascade depth gating (tether_mcp/tools/read_context.py): Added _cascade_depth counter tracked through _build_node_response → _add_children recursion. When N > 0 and _cascade_depth >= N, _add_children returns immediately without fetching children. This stops cascade traversal at N levels from the source node of the read_context call, preventing unbounded recursion under depth=-1. N=0 preserves existing unlimited behavior (backward compat).
• CoalescingBuffer auto-evict (interactive_agent_layer/coalescing.py): Added self.evict_expired() as the first line of record(), keeping the dict bounded to the last 5-second window without requiring an external caller.

What was already there (not changed)

• The per-child N tree-distance scope check via get_node_tree_distance (checks edges from conversation's current_node_id) is unchanged and independent of the new cascade depth gating.
• conversation_id requirement, M detail level, source filtering — all unchanged.

Tests

• 5 new unit tests in tests/tether_mcp/test_read_context_cascade.py (mocked DB): cascade stops at N, includes nodes exactly at N, N=0 is unlimited, depth param still respected, conversation_id required.
• 1 new test in tests/interactive_agent_layer/test_coalescing.py: expired entries evicted on record().
• 880 tests pass, 0 failures.

Test plan

• All existing tests green (880 passed)
• test_cascade_stops_at_N_depth_from_source: grandchild absent when N=1
• test_cascade_includes_nodes_up_to_and_including_N: child present at depth==N
• test_cascade_unlimited_when_N_is_zero: grandchild present when N=0
• test_cascade_respects_depth_param_independently: depth=1 still caps at 1
• test_evict_expired_called_on_record: old_tool gone after record() triggers eviction