fix(githubapp): continue reconcile past a default-branch 404

[balcsida]

Re-lands #103 by @team-humaki as a signed commit (the main ruleset requires signed commits; the original commit was unsigned). Authorship is preserved.

Closes #103
Fixes #102

What

Reconciler.All returned on the first DefaultBranchSHA error, so a renamed or deleted default branch skipped every later installation for the rest of the tick.

• Keep reconciling the rest of the installation and the remaining installations.
• Mark the failed repository with error_code=default_branch (cleared on the next successful read) and do not enqueue an empty SHA.
• Log the installation ID, owner/name, and HTTP status from the periodic refresh and webhook reconcile paths.

Schema / config / security

No migration: repositories.error_code is an unconstrained varchar(128). No config or security changes.

Verification

GOWORK=off go test -race -count=1 ./internal/githubapp/... ./cmd/graphnest-server/
make integration helm-test

Original PR CI (verify, integration, e2e, helm) passed.

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