feat(web): expose read-only terminal session history

[testikun]

Problem

Related to #347. The Web workbench has no way to discover compatible terminal/default Pi Sessions without copying them into Web storage or switching the active Web controller. This makes existing history invisible and risks conflating observer access with control.

Value

Provides a bounded, explicit read-only bridge to Pi's default Session source so users and future UI work can inspect terminal history while preserving Pi files and controller ownership as the authority.

Approach

• Add authenticated GET /api/terminal-sessions listing with bounded query, cursor, and limit validation.
• Restrict results to the currently selected, visible Web workspace and discover them from SessionManager.listAll() (Pi's default Session source).
• Add path inspection returning a bounded preview from the existing loadSessionPreviewData loader, including omission/byte evidence.
• Mark every result with source: "pi-default", origin: "terminal", and readOnly: true.
• Reject unavailable targets with an explicit 404 receipt; no copy, activation, mutation, editing, deletion, or fork path is introduced.

Validation

• Focused Web adapter/host tests: 34 passed, 0 failed.
• Full Node/Vitest suite: 1339 passed, 1 skipped, 0 failed; Vitest 30 passed.
• biome format / biome lint --error-on-warnings: passed.
• tsc --noEmit: passed.
• Config-contract, discipline-ledger, and Web syntax checks: passed.
• Ablation: removing the typed read-only-session error boundary changed missing-session responses from the required 404 SESSION_NOT_FOUND receipt to a generic 500; the boundary was restored.
• bun is not installed in this environment, so equivalent repository scripts were run with the bundled Node 24 executable and local Biome/Vitest binaries.

Impact

• User-visible behavior: new authenticated read-only terminal Session listing/inspection API; no UI changes in this PR.
• Model-visible context/tools: none.
• Runtime/lifecycle: active Web Session/controller is never changed.
• Persisted data: no Web index or Pi Session file is written.
• Compatibility/risk: output is bounded and source/read-only labeled; native UI integration remains a follow-up slice.