dotbot/cli: add --local-root to the device flash commands

[geonnave]

What

dotbot device flash-swarmit-sandbox and flash-mari-gateway gain --local-root, and every -f local flash now prints where each image actually came from.

Why

-f local reads ~/.dotbot/artifacts/swarmit-local/, a directory of symlinks into a build tree. Two problems in practice:

1. You cannot say which tree. The links are set by a separate dotbot fw fetch -S swarmit -f local --local-root <path> run, possibly days earlier. Working out of a git worktree, the obvious command (flash-swarmit-sandbox -f local --local-root <worktree>) was rejected with "No such option".
2. You cannot see which tree. The flash echoed the symlink path, not its target, so a link left pointing at a stale tree flashed silently. This actually happened here: the bootloader and netcore links pointed at a worktree while the two gateway links still pointed at another checkout with a build two months old.

Changes

• --local-root on both role-flash commands, passed through to flash_role, which re-links before flashing. Same flag name as dotbot fw fetch, so this is the existing two-step collapsed into one. Rejected with a clear message when --fw-version is not local, checked before the J-Link probe so it fails instantly.
• describe_image() resolves a symlinked image and appends the target plus its build timestamp:

[INFO] app hex: /Users/x/.dotbot/artifacts/swarmit-local/bootloader-dotbot-v3.hex
              -> /path/to/tree/device/bootloader/Output/dotbot-v3/Debug/Exe/bootloader-dotbot-v3.hex (built 2026-07-20 16:22)

A dangling link reports (BROKEN LINK) instead of the previous bare missing-file error.

Not done

_link_local_assets still maps all four assets from one root and fails if any is missing, so pointing --local-root at a tree that only builds the device images (no mari gateway) errors out. Linking per-role would fix that; it is a separate change.

Test

pytest dotbot/tests/test_device.py dotbot/tests/test_cli_deployment_fetch.py - 49 pass. Full pre-commit run --all-files clean. Exercised against real hardware artifacts: provenance output verified on live symlinks, guard verified to reject --local-root without -f local.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.50%. Comparing base (6f11e9c) to head (0df33ae).
⚠️ Report is 24 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #280      +/-   ##
===========================================
+ Coverage    83.27%   83.50%   +0.23%     
===========================================
  Files          119      121       +2     
  Lines        11120    11237     +117     
  Branches       570      570              
===========================================
+ Hits          9260     9384     +124     
+ Misses        1857     1850       -7     
  Partials         3        3              

Files with missing lines	Coverage Δ
dotbot/cli/device.py	84.21% <100.00%> (+0.69%)	⬆️

... and 2 files with indirect coverage changes

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