Drop blah2 tracker config and let it reach deployed nodes

[Purple10101]

Summary

Makes process.tracker.enable: false actually reach deployed nodes, and drops
the now-dead tracker config entirely. Pairs with
offworldlabs/blah2-arm#69, which removes blah2's built-in tracker.

The problem this fixes

config/default.yml has shipped process.tracker.enable: false since
2026-07-24 (46f7bc2), released in v0.4.2.0 and every tag since — its own
commit message cites memory growth as the reason.

It has reached zero nodes in nearly two months.

The merger seeds user.yml with a whole copy of default.yml on first boot,
and user.yml overrides default.yml. So every node on the estate has
enable: true frozen in its own overlay from whenever it first booted.
Confirmed on fairforest-b and owl-ded9: both merged configs said enable: true.

That tracker was leaking 2.00 MB/h — 100% of measured RSS growth, 18 days to
exhaust a 2 GB board with no swap. See the linked PR for the measurement.

migrate_remove_tracker() strips process.tracker and network.ports.track at
merge time. It joins the three existing migrate_*() functions that exist for
exactly this reason.

General lesson: changing config/default.yml alone changes nothing on any
deployed node. It needs a matching migrate_*() in merge_config.py.

Design notes

Unconditional, unlike the migrations that compare against a LEGACY_*
value to let a deliberate choice survive. There is no deliberate choice to
protect: the code these keys configured no longer exists, so every value is
equally dead.

Drops the keys rather than rewriting them, leaving user.yml on disk
untouched. That keeps the migration load-bearing rather than a one-shot fixup,
and lets a Mender rollback regenerate the old config from the old
default.yml. Verified live: after the merger ran on owl-ded9, user.yml was
byte-identical to a pre-migration backup.

Verified live on owl-ded9

Dropping first-boot process.tracker from user config: blah2's built-in tracker no longer exists
Dropping first-boot network.ports.track from user config: nothing publishes or listens on it any more

Both keys gone from the merged config, user.yml untouched, retina_tracker
sidecar config intact, and the node ran 12.8 h overnight with the paired blah2
image at 0.167 MB/h against the 2.00 MB/h it replaces.

Deployment: three images move together

config-merger, blah2 and blah2-api must be deployed as a set. Against
a config this migration has stripped:

• an old blah2 reads process.tracker.enable through ryml, whose default
  error handler calls abort() — it dies rather than warns. The docstring
  covers this.
• an old blah2-api calls server_tracker.listen(config.network.ports.track)
  with the key gone and binds a random free port, silently. This one is not
  in the docstring and is worth adding: during verification /api/tracker kept
  answering 200 after the strip, purely because the API had not restarted.

Merge note

This branch was 3 behind main and has been merged up. main added
migrate_adsb_truth_server() in the same two places this branch adds
migrate_remove_tracker() — a function body and a call in main() — and git
collapsed two similarly-shaped tests in test_merge_config.py whose "does not
rewrite user.yml" cases share a middle. Resolved by keeping both sides
everywhere. 53 tests pass, covering both migrations.

🤖 Generated with Claude Code

[Purple10101]

Pairs with offworldlabs/blah2-arm#69. Deploy config-merger, blah2 and blah2-api together: against a config this migration has stripped, an old blah2 aborts and an old blah2-api binds a random port.