* fix: ContextScout falls back to global core/ when local core/ missing
Users who install OAC globally have core standards at
~/.config/opencode/context/core/ but no local core/ directory.
ContextScout now does a one-time check on startup: if local
core/navigation.md doesn't exist, it checks the global path
from paths.json. This resolves the ENOENT errors when agents
try to read core/standards/ files that only exist globally.
- paths.json: enabled global path (~/.config/opencode/context)
- contextscout.md: added global_fallback rule with 2-glob cap
- Fallback is ONLY for core/ files, never project-intelligence
* docs: add context resolution explanation to README
Explains how ContextScout resolves context files: local-first,
global fallback only for core/ files, project-intelligence always
local. Includes flow diagram, key rules, and common setup table.
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