fix(cli): make the Python-subprocess timeout configurable via OPENANT_INVOKE_TIMEOUT

[gadievron]

What

Make the CLI's Python-subprocess timeout configurable via OPENANT_INVOKE_TIMEOUT
instead of a hardcoded 30 minutes.

Why

Invoke (apps/openant-cli/internal/python/invoke.go) bounded every Python
subprocess (parse/analyze/enhance/verify/report) with a hardcoded 30-minute
deadline and no override. A large repo whose analyze/enhance phase legitimately
runs longer than 30 minutes was killed mid-phase: the CommandContext deadline
fires, the watchdog closes the stdout pipe, and the in-flight io.Copy returns
read |0: file already closed. Because the phase writes its output only on
completion, the kill discards the whole phase's output and downstream phases
cascade on the missing file. The failure is volume-correlated — only repos large
enough to exceed 30 minutes hit it.

How

• resolveInvokeTimeout() reads OPENANT_INVOKE_TIMEOUT — a Go duration ("2h",
  "90m") or a bare integer of seconds — and falls back to the 30m default when
  unset, empty, or invalid (a warning is printed for a non-empty invalid value).
• The bare-seconds path is guarded against int64 overflow: a value beyond ~9.2e9
  seconds would wrap n*time.Second negative (an already-expired deadline that
  kills the subprocess immediately), so such values fall back to the default.
• The default is unchanged, so behavior is identical unless the operator opts in;
  the timeout still guards against a genuinely hung parser.

Tests

internal/python/invoke_timeout_env_test.go — the resolver parses the
duration/seconds forms, falls back on unset/invalid/overflow, and a wired test
proves the env value bounds the actual deadline of a real hung subprocess.
Existing invoke tests unchanged (go test ./internal/python/ green).

Compatibility

None. The 30m default is byte-identical when the env var is unset; completed
units are already checkpointed, so a re-run resumes.

Coordination

Open PR #235 (feat/webui) adds a parallel invoke path — InvokeCtx /
InvokeCtxCapture in a new file internal/python/invoke_ctx.go — for the web
server, using a caller-supplied cancellable context. There is no file overlap
with this PR (it does not touch invoke.go), so the two merge cleanly in any
order (verified: 0 conflict markers). Note the divergence for a follow-up: that
webui path does not honor OPENANT_INVOKE_TIMEOUT, so if both land, the CLI
Invoke() path honors the override and the webui path does not — unifying the
timeout handling across both is a reasonable follow-up once both are merged.