v0.19.0

Remove sem orient and all fuzzy/ranked retrieval (#463)

sem becomes **purely deterministic**. Removed: the `orient` command and
its `--pack` briefing, the sem-core ranking (lexical scoring, IDF,
recall net, structural priming), the `sem_entities query=` intent-search
mode, and the resident server's `orient` socket op.

**Why:** a 45-task validation (requests/flask/sphinx) showed orient's
~47% hit-rate could not be lifted by heuristics without causing
regressions (0 gains, 5+ regressions on a graph-expansion + path-penalty
attempt). Fuzzy ranking was the only non-deterministic thing in sem, and
it was where all the complexity accreted. The deterministic commands
never needed it.

**Remaining surface:** `context` (entity + callers/callees), `impact`
(blast radius), `diff`, `entities` (list by path, or `text=` for
exact-substring search), `blame`, `log`. To find code whose name you
don't know: text-search for a candidate, then `sem context` for the
structure grep can't give. The prompt-submit hook keeps its
deterministic exact-name prefetch.

745 lines deleted. Build clean, tests green (2 pre-existing `sem diff`
cwd-resolution failures unrelated).

Supersedes #462 (which added the fuzzy hook fallback this removes).

Note: sem-cloud has a `repo_orient` endpoint that pins a published
sem-core; it'll need the same removal when it bumps.