Fix stale README intro: add-on runs Device Builder by default

[esphbot]

What does this implement/fix?

The "Try it" intro still described the dashboard as an opt-in preview in
both the Home Assistant add-on and ESPHome Desktop. That's stale for the
add-on: as of ESPHome 2026.6.0 the add-on runs the Device Builder by
default (the "Home Assistant add-on" section just below already states this,
and the status banner is already marked stable/1.0). Only ESPHome Desktop
remains opt-in via the backend picker.

Reword the one intro sentence so it no longer calls the add-on a preview, while
keeping both paths and the "pick the path that matches how you run ESPHome
today" framing intact.

Related issue or feature (if applicable):

• fixes

Types of changes

• Bugfix (non-breaking change which fixes an issue) — bugfix
• New feature (non-breaking change which adds functionality) — new-feature
• Enhancement to an existing feature — enhancement
• Breaking change (fix or feature that would cause existing functionality to not work as expected) — breaking-change
• Refactor (no behaviour change) — refactor
• Documentation only — docs
• Maintenance / chore — maintenance
• CI / workflow change — ci
• Dependencies bump — dependencies

Frontend coordination

• No frontend change needed
• Companion frontend PR: esphome/device-builder-frontend#

Checklist

• The code change is tested and works locally.
• Pre-commit hooks pass (ruff, codespell, yaml/json/python checks).
• Tests have been added or updated under tests/ where applicable.
• components.index.json / definitions/components/*.json have not been hand-edited (regenerate via script/sync_components.py if a sync is needed).
• Architecture-level changes are reflected in docs/ARCHITECTURE.md and/or docs/API.md.

---

Quality Report

Changes: 1 file changed, 3 insertions(+), 2 deletions(-)

Code scan: clean

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan

[codspeed-hq]

Merging this PR will not alter performance

✅ 27 untouched benchmarks

---

_{Comparing esphbot:koan/readme-addon-default-not-preview (ff19be7) with main (3049402)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (3049402) to head (ff19be7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1549   +/-   ##
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  Coverage   99.54%   99.54%           
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  Files         224      224           
  Lines       17519    17519           
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  Hits        17439    17439           
  Misses         80       80           

Flag	Coverage Δ
py3.12	99.51% <ø> (ø)
py3.14	99.54% <ø> (ø)

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Pull request overview

Updates the README “Try it” introduction to reflect the current rollout status: the Home Assistant add-on runs Device Builder by default (no longer an opt-in preview), while ESPHome Desktop still requires explicitly selecting the backend.

Changes:

• Reword the “Try it” intro sentence to remove “opt-in preview” language for the Home Assistant add-on.
• Clarify that ESPHome Desktop uses an opt-in backend selection flow.