feat(web): refine agent and assistant surfaces

[GCWing]

Summary

• Consolidate the Agents overview into a compact catalog with source/kind filters and a clearer Harness progression presentation.
• Refresh Agent, Core Agent, and Assistant nursery cards while keeping responsive sizing, semantic tokens, and Appearance contracts aligned.
• Simplify the composer Harness trigger, align Agent terminology across locales, stabilize standalone component-preview appearance startup, and keep session-row tooltips anchored.

Type and Areas

Type: UI/UX / refactor

Areas: Web UI, Flow Chat composer, Agents scene, Assistant nursery, Appearance contracts, i18n

Motivation / Impact

The Agent and Assistant surfaces now present related capabilities with denser, more consistent hierarchy and interaction styling. Composer and gallery labels use the same Agent terminology, and the standalone component preview initializes a local built-in appearance before rendering.

Verification

Passed:

• Focused Vitest: 5 files, 47 tests
• pnpm run i18n:audit
• pnpm run theme:color-audit:all
• pnpm run appearance:contract-audit (passed with 16 existing shared-style owner warnings outside this diff)
• pnpm run motion:audit
• git diff --check origin/1.0.0-explore...HEAD

Known repository blocker:

• pnpm run type-check:web reaches the unchanged target-owned error src/infrastructure/api/adapters/websocket-adapter.ts(21,3): Module "@/generated/api" has no exported member "GitTrustReport". Neither the adapter nor generated API is changed by this PR.

Skipped:

• E2E, per request.
• Browser/native/manual visual validation was not run.
• Remote Workspace, Remote Control, Peer Device Mode, and Detached Dispatch were not exercised; this PR changes Web UI presentation only.

Reviewer Notes

• Rebased onto the latest origin/1.0.0-explore before submission.
• design-qa.md and todos-design-qa.md remain local and are not included.
• AI-assisted change; verification level is focused automated checks without E2E or manual visual validation.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.