feat(workspace-context): load .local. instruction overlays by default

Load a per-directory local overlay in addition to the base instruction
file, matching the Claude Code AGENTS.local.md / CLAUDE.local.md
convention for git-ignored personal guidance.

- New config `localInstructionFileCandidates`, default
  `['AGENTS.local.md', 'CLAUDE.local.md']`; empty disables the overlay.
  The default lives in the plugin Config schema, so every front door
  (TUI/ACP/headless) reads .local. files consistently.
- Per project directory the plugin loads the first-existing base
  candidate, then additively the first-existing local candidate,
  rendered after the base so it takes precedence within the byte budget.
- Base and local tiers get distinct scope keys via a NUL sentinel
  (scopeKey/decodeScopeKey) so they never collide in the baseline map,
  pending window, or version cache.
- The fixed user-global $DSH_HOME/AGENTS.md stays base-only.

Docs: README (config, lifecycle, Known Limitations), regenerated
config-catalog, and a new bilingual Agent Note cross-linked to the
owning workspace-context note. 100% per-file coverage retained.

docs/config-catalog.md

@@ -1728,10 +1728,12 @@ export interface Config {
   maxSourceBytes?: number
   /** Ordered same-directory project candidates; the first existing regular file wins in each scope. */
   instructionFileCandidates?: string[]
+  /** Ordered same-directory local-overlay candidates loaded in addition to the base file per scope; empty disables the overlay. */
+  localInstructionFileCandidates?: string[]
 }
 ```
 
-Source: [`packages/context/workspace-context/src/config.ts:16`](../packages/context/workspace-context/src/config.ts)
+Source: [`packages/context/workspace-context/src/config.ts:17`](../packages/context/workspace-context/src/config.ts)
 
 ## Loadable plugins with no config