Fix sidebar Space disclosure markers

[DeadWaveWave]

💡 Change Scope

• Small Change: Fast feedback, localized UI/logic, low-risk.
• Large Change: New feature, cross-boundary logic, runtime-risk (persistence, IPC, lifecycle, recovery).

📝 What Does This PR Do?

Fixes the sidebar Space affordance so expandability and rail identity are represented separately.

The previous implementation reused the disclosure icon for every Space. Empty Spaces therefore displayed a left-side arrow even though they had no child agents to expand, while removing that arrow wholesale made expandable Spaces ambiguous in the collapsed rail.

This PR:

• keeps the disclosure chevron for Spaces that contain agents;
• shows a subtle 6 px leaf marker only for empty Spaces in rail mode;
• removes disclosure affordances and aria-expanded semantics from empty Spaces;
• aligns empty-Space labels with other Space labels in expanded and peek modes;
• adds component and Electron E2E coverage for parent/leaf semantics, rail/peek/docked rendering, themes, and transition timing.

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🏗️ Large Change Spec (Required if "Large Change" is checked)

Not applicable — this is a localized Small Change with no new mutable state, persistence, IPC, or lifecycle behavior.

1. Context & Business Logic

A Space with agents is expandable and keeps a chevron. An empty Space is a leaf and gets only a passive rail marker.

2. State Ownership & Invariants

Rendering remains derived from the existing renderer-owned agent list.

• Only Spaces with agents expose a disclosure control and aria-expanded.
• Empty Spaces never imply expandability.
• Rail markers disappear cleanly as labels appear during peek/expand transitions.

3. Verification Plan & Regression Layer

Component tests lock down accessibility semantics. Electron E2E tests lock down geometry, themes, responsive sidebar modes, and transition behavior. The complete repository pre-commit gate passed.

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✅ Delivery & Compliance Checklist

• My code passes the ultimate gatekeeper: pnpm pre-commit is completely green.
• I have signed the CLA if required (project steward contribution; no separate signature required).
• I have included new tests to lock down the behavior.
• I have strictly adhered to the DEVELOPMENT.md architectural boundaries.
• I have attached a screenshot or screen recording (if this touches the UI).
• I have updated the documentation accordingly (CHANGELOG entry added in a separate commit; no feature or contract documentation changed).

📸 Screenshots / Visual Evidence

Playwright visual evidence was generated for light/dark themes and rail/peek/docked states. GitHub's browser upload surface was unavailable in this run, so the review-only contact sheet remains ready for manual attachment and is intentionally not committed to the repository.