Beads auto-detection requires a Beads database #103

Why

beads-autodetect made bd on PATH sufficient to auto-select the Beads
provider. On a developer machine bd is installed once, globally — so that rule
makes every repo look like a Beads project.

The consequence is not a cosmetic default. Reported from actual usage: a plain
specsync sync in a GitHub-tracked repo auto-selected Beads because bd was on
PATH — even though bd list reported no database — and would have created
phantom duplicate items instead of updating the issues specsync itself had
already created. The correct target was only reached by passing
-provider github explicitly. A default that quietly sends work to the wrong
tracker is worse than no default.

bd being installed says something about the machine. It says nothing about the
project. The project-level signal is a Beads database: a .beads/ directory.

What Changes

• BREAKING (to the auto-detection rule, not to any flag): bd on PATH alone
  no longer selects Beads. Auto-detection selects Beads only when a .beads/
  directory is present — at the resolved repo root or in the working directory —
  and bd is on PATH. A project carrying .beads/ on a machine without the
  binary falls back to github rather than failing on the first shell-out.
• Detection is anchored to the repo root derived from -openspec, not just the
  process working directory, so running specsync from a subdirectory resolves the
  same way.
• A .beads regular file is not a database and does not trigger detection.
• When auto-detection lands on a non-default provider, specsync prints the
  provider and the reason on real runs, not only under -dry-run. Where every
  item in a run is about to land is worth one line of output.
• -provider beads still selects Beads unconditionally; explicit always wins.

Impact

• Affected code: cmd/specsync/main.go (detectProvider, its runSync call
  site).
• Affected specs: beads — replaces the "Detect bd on PATH" requirement.
• Users who relied on bd-on-PATH auto-selection in a project with no .beads/
  now need -provider beads. That path could only ever have been driving a Beads
  database outside the project, which auto-detection was never able to locate.