fix(shell): discover devcontainer from any subdir of a worktree or repo

[rosstaco]

Problem

dcode shell (and dcode <path>) only looked at the exact target
directory for a .git pointer. Running from any subdir of a worktree
fell through to "no devcontainer.json found":

$ cd ~/repos/repo_a/.worktrees/branch_1/src
$ dcode shell
dcode: no devcontainer.json found for /…/branch_1/src; run `dcode doctor` to diagnose

The same blind spot silently affected plain repos: dcode . from a
subdir would fall back to plain code <subdir> with no container.

Fix

Two new helpers in core.py:

• _find_project_root(target) — walks ancestors looking for a .git
  (file or directory). Returns the directory containing it.
• resolve_target(target) -> (project_root, container_subdir) — the
  host directory that owns the devcontainer plus the path relative to
  the devcontainer's workspaceFolder that the user actually wants
  opened. For worktrees, project_root stays the main repo (all worktrees
  keep sharing one container) and container_subdir stacks the
  worktree-relative path and the user's subdir. For plain repos it's
  just the repo root + subdir. For non-repo targets it returns
  (target, Path('.')) so the existing "no devcontainer" fallback is
  unchanged.

run_shell, run_dcode, check_devcontainer, check_devcontainer_parses
and the doctor plan summary all switch to resolve_target so they
behave consistently from any depth, including dcode doctor which now
predicts what dcode . will actually do.

After

$ cd ~/repos/repo_a/.worktrees/branch_1/src
$ dcode doctor .
…
╭─ Plan for /…/repo_a/.worktrees/branch_1/src ─╮
│ Detected git worktree.                       │
│ Would anchor at the main repo /…/repo_a so   │
│ all worktrees share one container.           │
│ effective workspaceFolder:                   │
│   /workspaces/repo_a/.worktrees/branch_1/src │

Tests

333 passing, lint clean.

• tests/test_core.py: TestFindProjectRoot (5 cases),
  TestResolveTarget (5 cases including non-repo fallback),
  TestRunDcodeSubdir (3 end-to-end: plain subdir, worktree subdir,
  non-repo fallback).
• tests/test_shell.py:
  test_worktree_subdir_uses_main_repo_and_deeper_workdir (regression
  test for the reported bug, asserts find_container keys off the main
  repo and probe_workdir points at the deeper folder), plus
  test_plain_repo_subdir_resolves_devcontainer_at_repo_root.
• Existing test_worktree_uses_main_repo_for_lookup still passes
  unchanged (worktree-root behaviour is preserved).
• Three doctor tests updated for new wording ("project root" / "main
  repo" lowercase, and the external-worktree case which now reverts to
  the clean "no devcontainer" plan instead of an alarming "cannot be
  resolved" warning).

Out of scope

• No changes to container build / metadata logic — this is purely a
  path-resolution fix.
• check_worktree is unchanged; it's specifically a per-target sanity
  check (is THIS folder a worktree pointer file?) so widening it to
  walk-up would change its semantic.