Standardize import style (module-qualified vs direct-symbol) across the codebase

Context

Follow-up to #112, specifically the discussion started in this comment (import/naming inconsistency slows down both humans and LLM-assisted contributors trying to understand the codebase) and the detailed per-module audit in this comment (module-qualified vs. direct-symbol import style is mixed across knowledge_schema/kplib, secindex, convknowledge, propindex, reltermsindex, semrefindex, textlocindex).

AGENTS.md now documents the decision rule (added in this effort):

Default to Module-Qualified Imports: Prefer importing whole modules and using qualified calls to prevent namespace pollution, avoid name clashes, and provide immediate context for where functions or objects originate.
Use Direct Symbol Imports Cautiously: Restrict direct imports to specific scenarios: classes/exceptions/constants, avoiding severe repetitive clutter, or standard-library patterns.
Prohibit Wildcard Imports.

This issue tracks applying that rule consistently, based on an AST-based census of actual import sites across src/typeagent, tests/, and tools/.

Current state (as audited)

Plan

Phase 0: Land the AGENTS.md rule itself (docs-only).

Phase 1 — knowledge_schema/kplib cleanup (bounded, low-risk, directly matches the issue's original complaint):

• Convert the 20 from . import knowledge_schema as kplib + kplib.X sites to from .knowledge_schema import X, Y, Z, in src/typeagent (10 files) and in tests/tools (10 files, same mechanical fix).
• One PR.

Phase 1b: No code changes needed — secindex, convknowledge, textlocindex are already compliant/internally consistent.

Phase 2 (deferred, separate sign-off after Phase 1 lands): reltermsindex → propindex → semrefindex, each its own PR, in that order (ascending size/centrality — semrefindex touches add_messages.py and conversation_base.py, the highest-churn files in the repo per the code-health hotspot data, so it goes last after the mechanical pattern is validated on the smaller modules).

Scope decision: test/tool files get the mechanical kplib-alias fix (Phase 1) since it's zero-risk, but Phase 2's function-import style is not forced onto test files — importing a function directly to call it in an assertion is idiomatic pytest style and is being treated as its own justified exception, independent of what src/typeagent does.

Non-goals

This issue is about import style consistency only. Actually renaming modules (the original ask in #112, e.g. secindex → something more descriptive) is separate and still awaiting a draft rename list, per gvanrossum's request in #112.