Use lk agent dev instead of the deprecated Python dev command

[omarelsaid]

What

Replaces the starter's development command with the LiveKit CLI equivalent 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

uv run python src/agent.py dev now emits a deprecation warning:

dev mode is deprecated and will be removed in a future release; use lk agent dev instead

Since this is a starter template, the very first command a new user runs shouldn't be a deprecated one.

Notes for reviewers

• Scope is deliberately narrow: only dev mode is deprecated, so the console and start commands in the README and the uv run ruff / uv run pytest / Dockerfile / CI invocations are all left untouched.
• In the taskfile help output, {{ indent .INDENT "uv run" }} {{ .PYTHON_MAIN }} dev became {{ indent .INDENT "lk agent dev" }} — the whole command is now one literal, so .PYTHON_MAIN is no longer interpolated there. That variable is still used by the help_install_hint_if_needed check and the console help line, so it remains defined and in use.
• Verified: git diff --check is clean, agent.py dev no longer appears anywhere in the repo, lk agent dev appears in exactly the three intended locations, and taskfile.yaml still parses as valid YAML. The task binary isn't installed locally, so task dev itself was not executed — worth a quick sanity run by someone who has it.
• Users will need LiveKit CLI 2.15.0+ on their PATH for the new command; the README already covers installing the CLI in the "Using coding agents" section.

🤖 Generated with Claude Code

[omarelsaid]

Superseded by livekit-examples#100, which proposes the same change directly upstream.