fix(dev): default the command to headless mode

[Hweinstock]

Problem

• The default for dev is currently opening agent inspector
• the dev help shows tui, but doesn't support it.

Solution

• change default to headless
• update help text.
• remove tui support.

Verification

$ agentcore project create --name DevTest --template agent-python-strands 
$ cd DevTest
$ agentcore project dev & 
[1] 1401410
OTEL collector listening on port 43819; traces persist to [..]
[agent_python_strands] Starting development server
[agent_python_strands] INFO:     Will watch for changes in these directories: [..]
[agent_python_strands] INFO:     Uvicorn running on http://127.0.0.1:8082 (Press CTRL+C to quit)
[agent_python_strands] INFO:     Started reloader process [1401469] using StatReload
[agent_python_strands] INFO:     Started server process [1401497]
[agent_python_strands] INFO:     Waiting for application startup.
[agent_python_strands] INFO:     Application startup complete.
[agent_python_strands] Agent 'agent_python_strands' is running on port 8082.
$ agentcore project invoke runtime --local --port 8082 --payload '{"prompt": "hello world"}'
[agent response]

before this would open the browser.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

Small, focused fix. The three changes hang together: --mode enum drops tui, the always-throwing flags.mode === "tui" guard is removed, and the default flips to headless. InputValidationError is still used elsewhere in the file, so no orphaned imports. Nothing else in the repo references mode: "tui" besides a leftover in the test harness's helper type (src/handlers/project/dev/index.test.ts:169 still declares mode?: "browser" | "headless" | "tui"). That's harmless — no test actually passes "tui" — so not a blocker, but worth tightening in a follow-up.

One thing worth double-checking before merging: switching the default from browser to headless is a user-visible behavior change. The PR description notes the browser-open behavior is going away by default, which seems intentional given the help text update. Just make sure any docs/tutorials/release notes that show agentcore project dev opening the Agent Inspector are updated accordingly (I didn't find any in-repo, but external docs may exist).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.98%. Comparing base (81f5af2) to head (3d34600).
⚠️ Report is 1 commits behind head on refactor.

Additional details and impacted files

@@            Coverage Diff            @@
##           refactor    #2236   +/-   ##
=========================================
  Coverage     96.98%   96.98%           
=========================================
  Files           564      564           
  Lines         39226    39221    -5     
=========================================
- Hits          38042    38040    -2     
+ Misses         1184     1181    -3     

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