docs: correct the pre-flight checks section in how-it-works

[Paulo Lacerda (placerda)]

Corrects the Pre-flight checks section of docs/how-it-works.md, which
described a subsystem that does not exist as written.

This is the develop counterpart of #406. That PR fixed the copy on the
orphan docs branch, which is what the published site is built from. This
PR fixes the copy that lives alongside the code and is linked from
README.md and CONTRIBUTING.md.

What was wrong

Verified against the v0.8.6 tree.

1. Wrong module. The section pointed at pipeline/runtime.py. That
module only contains evaluator loading and per-row evaluator execution
(load_evaluator, run_evaluator, _model_config, _extract_score,
_is_transient_credential_error). The real implementation is
services/preflight.py, whose run_preflight() is imported only from
cli/app.py.

2. Wrong checks. The bullet list did not describe the checks that
actually run. The real display names are Workspace, Azure
authentication, Foundry project and Application Insights.

Negative evidence for the three documented checks that do not exist:

$ git grep -n "missing_packages\|required_env\|MISSING_ENV\|_check_env" -- src/agentops/pipeline
(no output, exit 1)

$ git grep -ln "socket\.\|create_connection" -- src/agentops
src/agentops/agent/sources/azure_monitor.py
src/agentops/cli/app.py

The only socket use in the CLI is _port_in_use, which checks whether the
cockpit port is already taken. It has nothing to do with resolving a URL
agent endpoint.

3. A flag that does not exist. The section told readers to use
agentops eval run --dry-run for CI gating. The real flags on eval run
are --agent --baseline --config/-c --format/-f --output/-o. The only
--dry-run in the whole CLI belongs to telemetry dashboard deploy.
Anyone copying that line into a pipeline gets a hard failure. The real gate
is --strict-preflight on agentops doctor.

Corroboration

docs/tutorial-prompt-agent.md already shows the real output:

AgentOps pre-flight  4 ok

covering the Workspace, Azure auth, Foundry project and App Insights
checks. The corrected text now matches that.

Also in this PR

• Adds the missing services/preflight.py entry to the documented
  directory tree.
• Repoints the "Where to Add New Code" row from pipeline/runtime.py to
  services/preflight.py.

How this was found

By resolving every documented agentops ... invocation against the real
click command tree and diffing the flags. That check now reports zero
suspect flags across README.md, src/agentops/templates/**/*.md and
docs/**/*.md on both this branch and the docs branch.