Add AWS SSM Parameter Store connector (boto3 extra)

[achadha235]

Summary

Adds an aws-ssm destination that syncs secrets/variables into AWS Systems Manager Parameter Store using boto3.

• Kind mapping: secrets → SecureString, variables → String
• Scope: required pathPrefix (joins with remote names for put/list/prune)
• Packaging: optional aws extra installs boto3; all pulls in every optional extra
• Auth: standard AWS credential chain (AWS_PROFILE / access keys) — no auth.tokenEnv
• Lazy import with a clear install hint when boto3 is missing
• Health check: separate skippable boto3 STS probe alongside the existing SST AWS CLI check

Config example

destinations:
  ssm:
    connector: aws-ssm
    region: us-east-1
deployments:
  - name: ssm-production
    set: production
    destination: ssm
    scope:
      pathPrefix: /myapp/production
    secrets:
      apiKey: API_KEY
    variables:
      logLevel: LOG_LEVEL

Install: uv tool install 'secretsync-cli[aws]' or 'secretsync-cli[all]'.

Test plan

• Unit tests for path join, kind→Type, validation, missing-boto3 error
• Integration tests with mock SSM client (put/list/delete batching, secret redaction)
• Contract capabilities + registry updates
• Full suite: 222 passed, 3 skipped
• Fixed CI: ruff format on aws_ssm.py (both ubuntu/macos format checks)