feat(ui): polish project sidebar agent presentation

[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?

Polishes the project sidebar agent presentation and agent title UX so dense workspaces stay scannable without changing cross-boundary behavior.

Highlights:

• turns each project agent list into a collapsible folder-style section
• compresses each sidebar agent row into a single task-first line with provider icon, space pill, and status dot
• supports double-click editing for the title suffix while keeping the provider prefix intact
• syncs label colors into sidebar agent rows and removes the extra right-side sidebar scrollbar regression
• refines provider branding, including the larger Claude Code flower-style icon
• updates CHANGELOG.md under Unreleased in a follow-up commit as required by repo workflow

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

N/A. This PR is a Small Change.

1. Context & Business Logic
N/A.

2. State Ownership & Invariants
N/A.

3. Verification Plan & Regression Layer
N/A.

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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 (see CLA.md).
• I have included new tests to lock down the behavior (or explicitly stated why it is untestable).
• 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 (if adding a feature or changing a contract).

📸 Screenshots / Visual Evidence

Local verification covered by:

• pnpm pre-commit
• tests/e2e/workspace-canvas.agent-sidebar.spec.ts
• tests/e2e/workspace-canvas.sidebar-workspaces.spec.ts