Release v0.1.2

Add typed errors, shared SSM workflow, and credential safety fixes

Error handling and reliability:
- Replace Box<dyn Error> with CloudShellError and CloudResult<T> in the
  library and CLI
- Add a 30s HTTP timeout to the shared reqwest client
- Retry only read-only calls (DescribeEnvironments, GetEnvironmentStatus)
  on connect/timeout errors and 429/5xx, up to 3 attempts
- VpcConfig::new no longer panics; validate() rejects blank IDs

Shared SSM and workflow code:
- Move connect, plugin check, and credential loading into src/ssm.rs
- Move get_or_create_env and wait_for_running into src/workflow.rs
- Remove the duplicated copies from main.rs and both examples
- Add a Progress trait; the library no longer prints. The CLI and
  examples pass StdoutProgress, and SilentProgress is available

Credential injection (connect --inject-credentials):
- Write credentials to a mode-600 file in /dev/shm, source it, delete it
- Use a trap to unset variables on exit instead of readonly
- Wait for a ready marker instead of fixed sleeps
- Remove the HISTFILE/HISTCONTROL settings, which gave no real protection
- Always delete the session and restore terminal mode on error (RawModeGuard)

Build and docs:
- Remove unused http dependency; keep aws-smithy-runtime, which provides
  the HTTPS client required at runtime
- Set rust-version to 1.99.0 and update README to match the current API
- Update examples to use the shared helpers