tool metadata: replace header comment parsing with .meta sidecar files
The tool registry now reads metadata exclusively from JSON .meta sidecar
files alongside each tool executable. Header comment parsing (ollie:prompt,
ollie:tier, ollie:parallel, args_json) is removed entirely.
- Add toolsrv/meta.go: MetaFile struct, LoadMetaFile, ToolInfoFromMeta
- Rewrite discover.go: scans .meta files only
- Rewrite schema.go: ParseToolInfo reads .meta only
- Rewrite tier.go: tier/readOnly from .meta only
- Generate .meta for all 35 tools
- Update all docs: README, AGENTS.md, ARCHITECTURE, CORE, USAGE, WRITING_TOOLS