feat: rename cwd to working-dir and add discover-path to integration-suites

[javierdelapuente]

Summary

Two improvements to integration-suites config:

1. cwd → working-dir

Renames the key that controls which directory pytest is invoked from. The new name is clearer and self-explanatory without knowing the implementation detail.

Before:

integration-suites:
  tests/integration/:
    cwd: ./

After:

integration-suites:
  tests/integration/:
    working-dir: ./

2. New discover-path key (closes #46)

Decouples the suite key (identity) from the directory used for test auto-discovery. This enables running the same test suite twice with different environments — e.g. juju 3.6 and juju 4 — without duplicating test files.

integration-suites:
  tests/integration/:
    working-dir: ./
    summary: juju 3.6 tests

  tests/integration/juju4/:
    discover-path: tests/integration/
    working-dir: ./
    summary: juju 4 tests
    environment:
      CONCIERGE: concierge-juju4.yaml

Here tests/integration/juju4/ need not exist on disk — it's just a unique suite identifier. Tests are discovered from tests/integration/ as usual.

discover-path combined with auto-discover: false raises ConfigurationError (they are mutually exclusive).

Changes

• src/opcli/core/spread.py: rename cwd key, add discover-path handling, extract _resolve_effective_discover_path helper (keeps C901 complexity in check)
• src/opcli/commands/pytest_cmd.py: update suite_cfg["cwd"] → suite_cfg["working-dir"]
• tests/unit/test_spread.py: update all fixtures/assertions; add 5 new discover-path tests
• tests/unit/test_pytest_templates.py: update cwd → working-dir in suite_config dicts
• examples/spread.yaml, AGENTS.md, docs/ISD283.md: updated docs