fix(tasks): reuse running process manager for process deps

[Arshgill01]

Fixes #3137.

devenv tasks run started devenv:processes:* dependencies in a fresh local runner even when those processes were already healthy under devenv up. That second copy crash-looped services such as PostgreSQL (initdb as root).

When a process manager is already running, tasks run now:

1. Collects the process names the task graph would schedule
2. Asks the native manager to start only missing names (Start / start_with_deps skips Ready/Starting/Waiting)
3. Waits until those processes are ready
4. Sets ignore_process_deps so the local runner does not launch them again
5. Does not stop_all on exit, so the existing manager keeps ownership

If no manager is running, behavior is unchanged: process deps start locally and are stopped when the task graph finishes.

Test plan

• cargo test -p devenv-tasks --lib scheduled_process_names
• cargo fmt --all -- --check
• Integration test tests/tasks-run-attach-running (devenv up -d then devenv tasks run test:repro must keep the dummy process pid and starts == 1)
• Manual: with services.postgres.enable = true and after = ["devenv:processes:postgres"], run devenv up -d; devenv tasks run test:repro without a postgres crash loop

[github-actions]

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Last updated: 2026-09-14T04:35:45.926Z

[Arshgill01]

Rebased onto latest main (includes #3146 and #3135). CHANGELOG keeps this PR’s #3137 entry plus main’s newer entries. behind_by 0, mergeable.

[Arshgill01]

Rebased this branch onto latest cachix/devenv main (6e830506). Conflicts were in CHANGELOG (moved the #3137 entry under 2.3.2 unreleased, kept newer main entries) and docs/src/content/docs/tasks.mdx (docs were converted from .md). Same head branch; no new PR.