Replace the it-module profile with the skipITs property

## Problem

The failsafe integration tests under `agent-module/plugins-it` are wired through the `it-module` Maven profile, which is `activeByDefault`.

- `activeByDefault` is dropped as soon as any other profile is activated on the command line (for example `-Pcode.coverage`), so whether `plugins-it` is part of the reactor depends on unrelated flags.
- A plain `./mvnw install` or `./mvnw verify` runs every plugin integration test, which is slow and not what a regular build needs.
- Skipping the tests requires knowing the profile name (`-P '!it-module'`) instead of the standard failsafe switch.

## Proposal

- Make `plugins-it` a regular module of `agent-module` and remove the `it-module` profile.
- Set `skipITs=true` in the root pom so the failsafe integration tests are skipped by default. Running them is the standard opt-in: `-DskipITs=false`.
- Update the IT workflows (`it-command.yml`, `it-schedule.yml`) to pass `-DskipITs=false` and drop `-Pit-module`. The release asset workflow only needs `-P '!agent-testweb-module'`.
- Document the commands in the testing guide.

```bash
# Integration tests (failsafe, verify phase; skipped by default)
./mvnw verify -DskipITs=false

# Plugin integration tests
./mvnw clean install -f agent-module/plugins-it -DskipITs=false
```

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