Add extension management to the Python SDK

[sanka-change-bot]

Summary

Add typed sync and async extension marketplace APIs, lifecycle configuration, complete structured CLI failure envelopes, and developer documentation for the local extension workflow.

Audit

• Requester: @hira29
• Authoring identity: Sanka Change Bot
• Head SHA: 2ae5e6add3932db8f1572aa7a41f2e39e74962b3
• Risk: medium
• Deploy mode: none
• Production surface: No production deployment; PyPI SDK publication is separately controlled.

Validation

• uv sync --frozen && uv run -- python -m unittest discover -s tests -v && uv run ruff check handwritten/sanka_sdk/migrate.py src/sanka_sdk/migrate.py tests/test_migrate.py && cmp handwritten/sanka_sdk/migrate.py src/sanka_sdk/migrate.py && uv build -> passed
• uv run --python 3.9 -- python -m unittest tests.test_migrate -v -> passed
• No-local-check reason: N/A
• Required GitHub checks must complete before landing.
• Valid review findings must be addressed before landing.

Change Safety

• Data or migration impact: No database or customer-data changes; SDK command construction, response decoding, and README documentation only.
• Rollback: Revert the merge commit and retain the previous sanka-sdk Python package release.
• Monitoring: Require current-Python and Python 3.9 tests, source parity, build checks, and GitHub Actions before approval.

Approval And Deployment Authorization

• The PR is ready for final human review; no intended code changes remain.
• The listed head SHA is the exact revision being presented for approval.

By approving this PR, the reviewer authorizes merge of the exact approved revision. This repository has no production deployment in this flow. A later push requires a new approval.