feat(dev): wire the Agent Inspector into project dev

[tejaskash]

What

Third and final PR of the Agent Inspector stack (C1 #2082 → C2 #2085 → C3). C1 built the Inspector HTTP layer and C2 added the agent-proxy routes, both tested in isolation behind fake dependencies. This PR wires that layer into agentcore project dev and adds the two IO leaves it needs, so the Inspector is reachable from the CLI for the first time.

Changes

• project dev is UI-by-default. New --ui flag (default true, so --no-ui is auto-generated) and --ui-port. In UI mode the handler resolves a UI port (explicit port must be free, else scans from 8081), starts the Inspector server via startHttpServer(createInspectorHandler({...})), hands the collector's TraceStore to it as InspectorTraces, watches agentcore.json to reload the supervised runtime set live, and opens the browser when interactive and not --json. --no-ui keeps the existing plain single-runtime log stream.
• src/io/openBrowser.ts — best-effort detached browser launch (open / cmd start / xdg-open), never blocks or fails the command.
• src/io/watchFile.ts — single-file watcher, debounced, closes on abort, fails quietly on a missing file or unsupported platform.
• OtelCollector.store renamed to traces so the collector hands its store to the Inspector without the Inspector knowing the collector exists. DevTraceCollector gains the matching traces accessor.

Why the two IO primitives belong in src/io

The handler is a real second caller of both, and both are raw platform mechanics (child-process spawn, fs.watch) that the boundary rule keeps out of src/core and the handlers. They live beside checkPort, startHttpServer, and the other IO leaves.

Lifecycle

The Inspector server rides the one AbortController shared with the collector, supervisor, and watcher, so Ctrl-C tears everything down through one cancellation domain. The collector closes only after runners return, so an agent's final spans still reach it. The config watcher's onChange swallows parse errors deliberately: a half-saved config parses on the next change event.

Tests

• project dev UI-mode suite: starts the Inspector and prints the URL, opens the browser only on a TTY and never in --json, /api/status lists every runtime with none started (lazy), agentcore.json edits reload the set, --agent narrows it, a taken --ui-port fails fast, --port with several runtimes is rejected.
• watchFile suite: debounces a burst of edits to one callback and stops on abort, a missing file fails quietly.
• All existing trace-collection and interruption tests kept, updated for the traces rename.
• bun test full suite green, typecheck / lint:check / format:check clean.

Verification altitudes

• Per layer: the suites above.
• End to end: pending on this branch after review — build the CLI, real project with two runtimes, confirm the browser opens the Inspector, invocations stream, MCP/A2A/resources answer, config reload is live, --no-ui gives the plain stream, --json never opens a browser.

Stacked on #2085. Merge C1 → C2 → C3 in order.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.67925% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.15%. Comparing base (2595b9d) to head (98e0b2a).

Files with missing lines	Patch %	Lines
src/io/openBrowser.ts	11.76%	15 Missing ⚠️
src/handlers/project/index.ts	72.72%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #2086      +/-   ##
============================================
- Coverage     97.22%   97.15%   -0.08%     
============================================
  Files           463      465       +2     
  Lines         28160    28301     +141     
============================================
+ Hits          27378    27495     +117     
- Misses          782      806      +24     

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[agentcore-devx-automation]

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

[agentcore-devx-automation]

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

[agentcore-devx-automation]

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