project: session move into an empty git repo re-points the global project's worktree

[stevenshack]

Summary

Moving a session (POST /api/session/{id}/move) into an empty git repo
(no commits, no remotes) does not create a project for that directory.
Instead the session stays in project global and the global project's
worktree is silently re-pointed at the target directory, making unrelated
directories cross-list each other's sessions.

Environment

• opencode version: 2.0.15
• OS: Linux 6.12.107+deb13-amd64 (Debian 13)
• Terminal: xterm-256color, truecolor
• Shell: fish
• Install/channel: latest
• Active plugins: none configured

Reproduction

1. Two directories, A and B, both empty git repos (git init, no commits, no
   remotes). Sessions from both were previously filed under project global
   (empty repos resolve to global by design).
2. opencode api post /api/session/<sesID>/move -d '{"directory": "<path-to-B>"}'
3. Inspect the project table: the global row's worktree has been
   rewritten from A to B.

Expected Behavior

The move should either create a project row for the target directory or leave
the session in global — and never mutate the global project record.

Actual Behavior

• The moved session remains in project global.
• global's worktree is re-pointed at B. Every directory that resolves to
  global (all empty repos) then shows the same mixed session list: two
  unrelated projects' sessions appear in each other's /sessions.
• Recoverable via PATCH /api/project/global -d '{"canonical": "/"}', which
  restores the catch-all semantics.

Additional Context

• Underlying cause: empty repos have no derivable identity (no root commit,
  no origin remote), so resolution falls back to global, and the
  session-adoption step in the project bootstrap skips global by design.
• Suggested fix directions: create a project for the move target (or reject
  the move with a clear error), and treat the global record as immutable.
• Workaround that made the directories first-class: create an initial commit
  in each repo (root-commit SHA becomes the derived project identity).