Docs: Chez-only, drop the Janet-era references and obsolete migration notes

Bring the docs in line with the actual implementation now that Chez is the sole
substrate.

Deleted the migration/spike/handoff artifacts that only documented the Janet
era or the port effort: the port plan, phase-0 and foundational-runtime spike
writeups (+ the stray root-level copy), the self-hosting design notes, the
architecture-refactor plan, and spike/chez/RESULTS.md.

Rewrote the current reference docs against the Chez facts: building-and-deps and
tools-deps (no jpm/build step — bin/joltc off the checked-in seed, deps via
jolt.deps into ~/.jolt/gitlibs), libraries (SQLite is built-in jdbc.core over
libsqlite3, not a Janet driver), the conformance/spec test-flow docs (the Chez
corpus runner + certify, no .janet harnesses), and the transient / type-hint /
seed-overlay design notes (Chez representations: mutable transients, flat
copy-on-write vectors, HAMT maps, the seed/overlay twin). Fixed the README
collections line (vectors aren't 32-way tries) and added the ffi/transient gate
targets. rfc 0001's numerics open-question is resolved (the Scheme tower).

Renamed the built-in HTTP adapter to jolt.http.server only (dropped the
ring-janet.adapter alias — a Janet-era name).

docs/rfc/0001-language-specification.md

@@ -170,9 +170,9 @@ Signature(s), since-version
 ## Open questions
 
 1. Numerics: the reference has longs/doubles/ratios/BigInt with promotion
-   rules; CLJS has JS numbers; jolt has Janet numbers. Likely answer: specify
-   an integer/float core with a host-numeric-tower extension point — needs
-   its own design note in §4.
+   rules; CLJS has JS numbers. Resolved: jolt carries the Scheme numeric tower
+   (exact integers/bignums, exact ratios, flonum doubles), matching the
+   reference's tower — see the numerics note in §4.
 2. Where do `*print-length*`-style dynamic vars land — host-dependent
    interface or portable with defaults?
 3. License/venue if the spec outgrows this repo (likely CC-BY; separate repo