refactor(chat): remove auto file and dir attachment

[cniska]

Motivation

Auto-inlining @ file and directory contents burned context tokens before the model decided what was relevant. Letting the model pull what it needs through existing tools aligns with the on-demand retrieval pattern already used by memory.

Summary

• replace file content injection with suggestions via req.suggestions
• suggest code-scan for parseable code files, file-read for other files, file-find for directories
• remove isRelevantFileContext and attachment message token budget from agent input
• remove maxAttachmentMessageTokens from lifecycle policy and constants
• unresolved @ refs now warn but no longer block the turn
• export isParseable from code-ops for reuse

Fixes #196