docs(flagd): cut the adoption README to what only it can say

16504 bytes to 6157. An adoption README answers three questions: what
this provider declares and why each absence is what it is, what the
tally is and which failures are expected, and the one command that runs
it. Everything else here was the harness README's or Appendix F's.

Deleted, with a pointer in its place:

  - the build-tag/module-path mechanism, ~70 lines arguing why a tag is
    not an exclusion, why TCK_RUN was removed, why testing.Short is not
    the gate and why the old -run 'Conformance' filter went. All of it
    is identical in the OFREP adoption and in tools/tck/README.md, and
    the reasoning above it is Appendix F's "Running the suite in CI".
  - the argument for a conformance suite being a sibling of the e2e
    suite rather than a package inside it -- Appendix F's, stated at
    length there and now linked.
  - the restatement of Appendix F's first rule for declaring, twice.

The per-capability reasoning is the one thing worth keeping at length,
and it already lives in the comments beside each declaration in
tck_test.go, which is the better home: the file a reader is in when they
change a declaration. The README now carries a one-line summary per
capability and says where the full version is.

Kept in full because it is this adoption's own measurement: the tally
and its four expected failures, the split between the provider's defect
and the fixture's three, and the six-run range that says to re-run a red
result before reading anything into it.

No behaviour change and no count moves.

Signed-off-by: Simon Schrottner <simon.schrottner@flagsmith.com>

Author: aepfli