chore(deps): migrate z3 bundled feature to vendored

[purp]

Description

z3 0.21 deprecates the bundled feature in favor of vendored. z3-sys
0.13's build script emits cargo:warning=The 'bundled' feature is deprecated. Please use the 'vendored' feature. on every build that enables it.

OpenShell wires the deprecated name through its own feature aliases:

• crates/openshell-prover/Cargo.toml:14 — bundled-z3 = ["z3/bundled"]
• crates/openshell-prover-cli/Cargo.toml:18, crates/openshell-server/Cargo.toml:123,
  crates/openshell-gateway/Cargo.toml:75 forward bundled-z3 onward

The work is to decide whether to repoint the bundled-z3 alias at
z3/vendored, and whether the public-facing alias name should change with it.

Context

Both features currently resolve to the same z3-sys code path, so nothing is
broken today and there is no user-visible behavior change. This is
forward-maintenance: the deprecated name will eventually be removed upstream.

This path is not incidental — it is the release build path. tasks/ci.toml:34
and tasks/scripts/stage-prebuilt-binaries.sh:172-173 force bundled-z3 for
the gateway, so tagged releases compile Z3 from vendored source through it.
That makes the change low-urgency but not zero-risk, which is why it was kept
out of the z3 0.21 upgrade (#3552) and its documentation follow-up.

bundled-z3 also appears in contributor-facing docs (README.md:362,
CONTRIBUTING.md:312-363) and in .agents/skills/build-openshell-mxc-windows/SKILL.md,
so a rename would need those updated in the same branch.

Not required for 0.1.0.

Definition of Done

• A decision is recorded on whether to repoint bundled-z3 at z3/vendored
  and whether to rename the alias
• If repointed, the gateway release build and the prover test suite are
  verified against the vendored path
• Contributor docs and the Windows build skill match the resulting feature
  names
• No deprecation warning is emitted for the Z3 source-build path