feat(architecture): add a conventions lens to /architecture:improve #11126

Refs: #7590

Agent Brief

Type: Feature
Summary: /architecture:improve gains a conventions lens that derives the codebase's building blocks, owners and feature blueprints and proposes rules, recording none without a yes.

Current behavior:

• plugins/architecture/skills/improve/SKILL.md has two lenses, deepening and boundaries (actions/deepening.md, actions/boundaries.md). Its "Adding a lens" section says a new lens is an actions/<lens>.md playbook, a research/<lens>/ folder and one Actions row.
• The four agent hazards and the one-paragraph ADR shape shipped in feat(architecture): add four agent hazards to the deepening scan and a one-paragraph shape to record-decision #8646. The boundaries lens exists.

Desired behavior (from #7590's decisions, D65, Q47, Q576):

• A conventions lens, added per "Adding a lens": it derives building blocks, owners and feature blueprints from the code and proposes rules.
• It records no rule without the person's yes. Routing of what it finds: boundary rules to the architecture concern file; findings to /corrections:codify; structural paved-path entries to /discipline:reuse-or-replace.

Done when

• /architecture:improve documents the conventions lens: an Actions row, actions/conventions.md and research/conventions/.
• Eval cases cover: a proposal made and nothing written without a yes; a boundary rule routed to the architecture concern file; a finding routed to /corrections:codify; a paved-path entry routed to /discipline:reuse-or-replace.
• bash plugins/skill-quality/scripts/check-skill.sh plugins/architecture/skills exits 0.
• scripts/check-changelog-fragments.sh passes with an architecture fragment.

Out of scope:

• A generated project skill, bundled per-stack import adapters, a Dune-like framework.