remove string hints — consolidate to hints_structured with reason field

Remove the redundant `hints: list[str]` field from all five MCP tool
output models. Consolidate advisory text into a new `reason: str` field
on StructuredHint. Eliminate dual-emission maintenance burden and the
parity test.

- Remove all TPL_* string template constants from mcp_hints.py
- Remove finalize_hint_list, MCP_HINTS_FIELD_DESCRIPTION, and
  string-only helper functions
- Add reason: str = "" to both _StructuredHint and StructuredHint
- Change generate_hints return type to list[_StructuredHint]
- Remove hints: list[str] from SearchOutput, FindOutput,
  DescribeOutput, NeighborsOutput, ResolveOutput
- Update server.py tool descriptions to reference hints_structured
- Migrate all test assertions from out.hints to out.hints_structured
- Add test_structured_hints_reason_char_cap and test_no_string_hints_field

Implements: plans/PLAN-HINTS-STRING-REMOVAL.md (PR-1)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: HumanBean17

docs/AGENT-GUIDE.md

@@ -16,7 +16,7 @@ Copy the block between `<!-- BEGIN` and `<!-- END` into your project's `AGENTS.m
 
 **Ontology: 15** — if results look structurally wrong or empty across tools, the index may be missing, stale, or built with a different `ontology_version`; you cannot re-index via MCP — ask the operator to rebuild.
 
-**Responses:** On success, `search`, `find`, `describe`, `neighbors`, and `resolve` may include a top-level `hints` list (≤5 suggested next calls) and a companion `hints_structured` list of `{tool, args, actionable}` objects — machine-parseable equivalents with ready-to-use parameters (`actionable=true`) or advisory signals (`actionable=false`). For `search`/`find`, echoed `limit`/`offset`. Hints are advisory; ignore them when `success` is false.
+**Responses:** On success, `search`, `find`, `describe`, `neighbors`, and `resolve` may include a top-level `hints_structured` list (≤5 advisory next-step objects). Each element has `{tool, args, actionable, label, reason}` — machine-parseable with ready-to-use parameters (`actionable=true`) or advisory signals (`actionable=false`, with a `reason` explaining why). For `search`/`find`, echoed `limit`/`offset`. Hints are advisory; ignore them when `success` is false.
 
 **Use this MCP when** you need whole-codebase structure: callers/callees, route handlers, HTTP/async seams, clients/producers, or fuzzy entry points for a concept.