Add extensions framework for custom btcli plugins

[tmimmanuel]

Summary

Implements the extensions system proposed in #500. This allows developers to create, install, and manage custom btcli plugins without modifying the core codebase.

What changed

• Added btcli ext command group with subcommands: add, update, remove, list, create, test, run
• Extensions are stored in ~/.bittensor/extensions/ and validated via an extension.yaml manifest (name, version, description, entry_point)
• ext create scaffolds a new extension with manifest, entry point, and test template
• ext run resolves the entry point from the manifest, avoiding hardcoded paths
• Aliases btcli extension and btcli extensions are also registered

Why

Currently there is no way to extend btcli with custom commands. This framework provides a standard structure for community-built plugins while keeping the core CLI clean. See #500 for the original proposal and discussion.

New files

• bittensor_cli/src/commands/extensions/ — extension management commands, manifest parsing, templates
• tests/unit_tests/test_extensions.py — 19 unit tests covering manifest parsing, directory helpers, and CLI commands

Test plan

• btcli ext create test-ext creates boilerplate at ~/.bittensor/extensions/test-ext/
• btcli ext list shows the created extension
• btcli ext run test-ext prints "Hello from test-ext!"
• btcli ext test test-ext runs and passes the scaffold test
• btcli ext remove test-ext removes the extension
• btcli ext add <valid-repo-url> clones and validates manifest
• btcli ext add <repo-without-manifest> fails gracefully and cleans up
• btcli ext update pulls latest for all installed extensions
• btcli extensions list and btcli extension list work as aliases
• pytest tests/unit_tests/test_extensions.py — all 19 unit tests pass

Closes #500

[thewhaleking]

Read CONTRIBUTING.md

[tmimmanuel]

@thewhaleking I am really sorry for my mistake.
I read the CONTRIBUTING.md again deeply one line by one line.
Could you please reopen my PR? I will target to staging branch again.
Also I will write some more commits to enhance my PR.

[thewhaleking]

I think overall this is going in the right direction, but I think it needs to finish its thought.

I think this is a good first step, but needs to be a bit better fleshed out. Ideally I would like to see a test extension that does something.