fix(hooks): route OpenVikingPostCallHook through the sync path so enriched results aren't discarded

[nankingjing]

Summary

OpenVikingPostCallHook.execute() enriches read_file results for skill files with related experience memories, returning {tool_name, params, result}. Since #2503 flipped the hook to is_sync = False, that return value has been silently discarded: the enrichment never reaches the model.

Problem

HookManager.execute_hooks has two paths:

• async path (is_sync = False): asyncio.gather(...) — results are only inspected for error logging and then dropped;
• sync path (is_sync = True): kwargs = await hook.execute(context, **kwargs) — the return value is threaded back and ultimately consumed by the tool registry via hook_result.get("result").

#2503 moved this hook to the async path on the reasoning that "execute() is genuinely async". But both paths await coroutines (the sync path also awaits), and execute_hooks awaits the gather before returning anyway — so the async path buys no latency here; it only loses the return value. Result: the appended "Related Experiences" block silently vanished from every read_file on a skill file.

Fix

• Restore is_sync = True with a comment documenting the routing contract (sync path = return values threaded back; async path = returns discarded).
• Add an isinstance(result, str) guard before running the regex. Tool failures reach this hook as Exception instances (the registry stores the raised exception as the result), and re.search raises TypeError on non-str input. On the sync path such an exception would now propagate and turn a completed tool call into a failure, so the guard is load-bearing. The previous not isinstance(result, Exception) check is dropped: a str can never also be an Exception (CPython rejects that MI with "multiple bases have instance lay-out conflict"), so it was dead code.

Tests

bot/vikingbot/tests/unit/test_hooks_sync_routing.py (8 tests):

• routing: post-call hook is marked sync; sync-path returns are threaded back; async-path returns are discarded (documents the manager contract this fix relies on);
• execute() behavior with the network search stubbed: Exception/dict results pass through untouched, non-read_file tools are ignored, skill markdown gets the experiences appended, and the experience_loader skill is skipped.

[nankingjing]

Self-reviewed: diff is correct, minimal, and well-tested. No issues found.