feat(skill-writer): Add scope-aware encapsulation depth gate

[the-simian]

Add a Scope-Aware Encapsulation principle to skill-writer/references/design-principles.md, a matching hard-stop rule in mode-selection.md, and a sister-update to skill-writer/SPEC.md, so authored skills stay matched to the level at which they are registered.

Why. skill-writer's synthesis path explicitly permits local project files as grounding evidence. For a project-scoped skill, surfacing those files' identifiers in the authored body is correct; for a globally registered skill, it is not. The rule is implicit in good practice but is not currently codified anywhere skill-writer consults: Independence And Portability covers provider portability (paths, runtime deps), and mode-selection.md's portability note in synthesis outputs covers provider-specific mechanics, not registration-scope encapsulation of body content. An authoring agent following the documented synthesis-then-author flow can therefore land a global skill whose worked examples reference symbols that exist in only one project, producing references that resolve to nothing for every other consumer of the skill. This PR adds the principle, the matching hard-stop, and the contract bullet.

Scope of the principle. The rule targets body content vs. registration scope: a skill's worked examples and inline references must not exceed the scope at which it is registered. Source synthesis may consult a project's files to learn the canonical pattern; the resulting body must abstract that pattern with structural placeholder names. Project-scoped skills are explicitly carved out and may anchor freely on real codebase symbols.

What changed.

• skills/skill-writer/references/design-principles.md: add a Scope-Aware Encapsulation section after Independence And Portability with a registration/scope/body-may-reference table, an explicit one-sentence distinction from the prior section, and four bullets covering the resolution rule, the abstraction rule for globals, the consult-but-don't-bake rule for synthesis, and the carve-out for project skills.
• skills/skill-writer/references/mode-selection.md: add one line to Hard stop rules flagging hard-coded project-specific identifiers in a globally registered skill as a hard failure. Cross-references the new section.
• skills/skill-writer/SPEC.md: add one bullet to Non-negotiable constraints mirroring the new principle, so the skill's maintenance contract reflects the new gate.

[the-simian]

Force-pushed a few times to fold self-review fixes into the original commit (current HEAD 74b1a91). The Sentry bot's MEDIUM finding on placeholder names is addressed. Diff is +18 / -0 across the three files in the PR body.