Use lk agent dev instead of the deprecated Python dev command

[omarelsaid]

What

Points the starter's development instructions at the LiveKit CLI, in three places:

• README.md — the dev snippet under "Run the agent"
• taskfile.yaml — the help output printed by the web console flow
• taskfile.yaml — the dev task itself

Why

Running the documented command emits two deprecation notices:

dev mode is deprecated and will be removed in a future release; use lk agent dev instead
in-process auto-reload has been removed from the Python CLI; use lk agent dev for hot-reload

The second one matters most: hot-reload is gone from the Python CLI entirely, so lk agent dev isn't just a rename — it's now the only way to get reloading. As this is a starter template, the first command a new user runs shouldn't be a deprecated one that silently gives them less than the docs imply.

Notes for reviewers

• Scope is deliberately narrow. Only dev mode is deprecated, so the console and start commands in the README, the uv run ruff / uv run pytest invocations, the Dockerfile, and the CI workflows are all untouched.
• In the taskfile help output, {{ indent .INDENT "uv run" }} {{ .PYTHON_MAIN }} dev became {{ indent .INDENT "lk agent dev" }} — the command is now a single literal, so .PYTHON_MAIN is no longer interpolated at that line. It remains defined and in use by help_install_hint_if_needed and the console help line.
• The new command needs LiveKit CLI 2.15.0+ on the user's PATH. The README already covers installing the CLI under "Using coding agents", so I didn't duplicate that here — happy to add a pointer next to the command if you'd prefer one.
• Verified: git diff --check clean, agent.py dev no longer appears anywhere in the repo, lk agent dev appears in exactly the three intended places, and taskfile.yaml still parses as valid YAML. I don't have task installed locally, so task dev itself hasn't been executed end to end.

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