refactor(config): simplify loading and fallbacks

[kitlangton]

What

Centralize well-known manifest loading and make Config's object narrowing explicit. Constant filesystem fallbacks now use Effect's dedicated orElseSucceed combinator without changing which failures are recovered.

How

• Reuse one local WellKnown.loadEntry workflow across load, refresh, and add.
• Replace repeated object checks and record assertions in config diffing with isRecord.
• Replace six constant Effect.catch fallbacks in skill discovery with Effect.orElseSucceed.

Scope

Behavior-preserving loading cleanup only; no config format, diagnostics, manifest, watch, or recovery-policy changes.

Testing

• Targeted config, well-known, and skill discovery tests (54 passed)
• bun typecheck from packages/core
• Push hook: bun turbo typecheck --concurrency=3 (40-package workspace)

[kitlangton]

Consolidated into #43930 so the Plugin and configuration boundary cleanup can be reviewed together.