dotbot: README swarm-first rework + fetch defaults to latest

[geonnave]

Two onboarding improvements for the conference, on one branch (kept together by request).

README / docs: lead with the swarm path

The README now flows ## Usage -> ## Quickstart - a swarm -> ## Going further. The one-bot walkthrough moved into a new doc/guides/one-bot.md; the LH2 quickstart already lived in the LH2 guide, so it's just removed from the README. ## Going further links both guides and now includes the config command (previously dropped from the "four namespaces" line). The device and config-reference docs also gained the per-machine vs per-project segger_dir/firmware_repo split.

fw fetch / flash: default to the latest release

Bare dotbot fw fetch (and flash-mari-gateway / flash-swarmit-sandbox with no -f) resolve the latest swarmit release via the GitHub /releases API - prereleases included, since the current newest is an rc - print the resolved tag, and cache under it. -f <tag> still pins; -f local is unchanged. The resolution happens in the CLI layer (before flash_role) so the concrete tag, not the literal "latest", drives the cache dir.

Fetch output is now package-manager style: a "no version specified, fetching latest: " line, clean per-file names + sizes, a one-line optional-skip summary, and relative paths - instead of the raw [GET]/[OK]/404 noise.

Validation

Host-side: the resolver (live + mocked unit tests) and an end-to-end fetch, 79 tests green with no network calls in the suite, docs build clean under -W. The role commands' on-bench flashing is unchanged logic (only the version now defaults to latest) - worth a quick bench confirm.

A follow-up will add a second fetch source (DotBot-firmware bare apps for a no-SES one-bot path) once that repo's release CI publishes them.

[codecov]

Codecov Report

❌ Patch coverage is 78.42105% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.27%. Comparing base (6c8db27) to head (f93abc8).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
dotbot/cli/_artifacts.py	25.92%	20 Missing ⚠️
dotbot/examples/circle/circle.py	30.43%	16 Missing ⚠️
dotbot/cli/fw.py	86.36%	3 Missing ⚠️
dotbot/cli/device.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #270      +/-   ##
===========================================
- Coverage    83.35%   83.27%   -0.08%     
===========================================
  Files          118      119       +1     
  Lines        10963    11120     +157     
  Branches       570      571       +1     
===========================================
+ Hits          9138     9260     +122     
- Misses        1822     1857      +35     
  Partials         3        3              

Files with missing lines	Coverage Δ
dotbot/cli/demo.py	100.00% <100.00%> (ø)
dotbot/cli/main.py	100.00% <100.00%> (ø)
dotbot/config.py	98.02% <ø> (-0.02%)	⬇️
dotbot/tests/test_cli_helpers.py	100.00% <ø> (ø)
dotbot/tests/test_device.py	100.00% <100.00%> (ø)
dotbot/cli/device.py	83.51% <75.00%> (-0.83%)	⬇️
dotbot/cli/fw.py	90.60% <86.36%> (+1.79%)	⬆️
dotbot/examples/circle/circle.py	30.43% <30.43%> (ø)
dotbot/cli/_artifacts.py	33.33% <25.92%> (-6.07%)	⬇️

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[geonnave]

This is probably ready, but needs more testing with real hardware.

[geonnave]

The README was validated by @RasdaCorentin, with improvements noted at #272. Nothing prevents merging this now, and improvements can land later.