fix(mise): update bootstrap installer

[shunk031]

Summary

• update the pinned mise bootstrap version from v2026.3.7 to v2026.6.13
• use the release install.sh asset instead of the raw install.envsubst template
• clear mise installer env vars before running mise install

Context

The scheduled Snippet install workflow failed in run 28215014929 during mise install. The old pinned mise build could not resolve the current antigravity-cli aqua registry entry and selected the old asdf sqlite plugin path, which attempted to download an outdated SQLite archive.

Validation

• shfmt -i 4 -sr -d install/common/mise.sh
• shellcheck install/common/mise.sh
• bash -n install/common/mise.sh
• rendered home/.chezmoiscripts/common/run_once_after_02-install-mise.sh.tmpl with chezmoi and checked it with bash -n
• verified mise v2026.6.13 resolves sqlite and aqua:google-antigravity/antigravity-cli versions

Note: bats tests were not run locally per repository policy.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.88%. Comparing base (78a8b82) to head (488ee39).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #474      +/-   ##
==========================================
- Coverage   89.69%   88.88%   -0.82%     
==========================================
  Files           9        9              
  Lines         233      234       +1     
==========================================
- Hits          209      208       -1     
- Misses         24       26       +2     

Flag	Coverage Δ
macos-14-client	41.66% <ø> (ø)
ubuntu-latest-client	88.88% <100.00%> (+0.09%)	⬆️
ubuntu-latest-server	86.63% <100.00%> (-2.16%)	⬇️

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