Improve session capability detection

[bifrost0x]

What changed

• collect session metrics with guarded POSIX and BusyBox-compatible probes
• render only metrics and diagnostic sections that returned usable data
• keep base telemetry active when optional expanded diagnostics are unsupported
• probe SFTP capability on a fresh, bounded channel before auto-opening the embedded SFTP pane
• retry transient SFTP probe failures without treating them as permanent incompatibility
• preserve early terminal output and switch or appliance prompts during attachment and restore

Why

Servers, switches, and appliances expose different command sets and SSH subsystems. Empty metric panels and an automatically opened but unusable SFTP pane created unnecessary errors for unsupported targets. The session workspace now adapts to the capabilities actually available while keeping probes bounded and read-only.

The standalone SFTP Manager is unchanged, and no SCP fallback is introduced.

Validation

• pytest tests -q: 1603 passed, 33 skipped
• npm run test:js: 216 passed
• npm run test:e2e: 6 passed
• npm run lint:js
• npm run vendor:check
• git diff --check

Implements the compatibility behavior discussed in Discussion #116.