Audit snapshot uploader scope: dotfiles under dataDir now writable via HOME

[bdchatham]

Problem

After #234, `HOME` on the seid container points at `dataDir` (`/sei` today). Cosmos SDK / seid subcommands that consult `~/.foo` will now write into the data PVC instead of failing with ENOENT/EACCES. Examples:

• `~/.bash_history` if anyone `kubectl exec`s with a shell
• `~/.cache/` from any Go binary using `os.UserCacheDir()`
• Cosmos SDK keyring backends defaulting to `~/.seid/keyring-*` (mediated by sidecar today, but a future image with debug entrypoints could trigger)

Surfaced during cross-review of #234 by the security-specialist agent.

Impact

The snapshot uploader (in the sidecar, `seictl serve`) is the load-bearing question. If it tars `/sei` wholesale, any dotfile that ends up under dataDir rides along into S3 and into every node that restores from that snapshot. This is "silently-introduced persistence surface" — no exploit today, but expands threat surface and creates a confused-deputy risk for future seid plugins.

Relevant experts

• security-specialist (threat model)
• platform-engineer (snapshot pipeline ownership)

Proposed approach

Two layers:

1. Audit the sidecar's snapshot upload code path. Find what subdirectory of `dataDir` is tar'd. If it's a specific subdir (e.g., `data/`), this issue is informational only — close after documenting. If it's wholesale `dataDir` content, add explicit excludes.

2. Add a defensive comment block in `internal/noderesource/noderesource.go` near the HOME env declaration, calling out that `dataDir` now resembles a home directory and any future tooling that tars or syncs from there should be explicit about what's included.

3. (Cut-first) Default excludes in the uploader: `.bash_history`, `.cache/`, `.ssh/`, `.aws/`, `.kube/`, `.config/`, `keyring-*`. Only relevant if step 1 shows wholesale-tar behavior.

Acceptance criteria

• Sidecar snapshot uploader scope is documented (in this issue or a doc reference)
• If wholesale-tar: excludes added, tests added, follow-up tracked
• If scoped-subdir: this issue closed with link to the relevant sidecar code

Out of scope

• Migrating dataDir to a structure that more cleanly separates "home dir" from "data dir" (would be a much larger refactor; the convention everywhere is `~/.sei == dataDir`)
• Cosmos SDK keyring relocation

References

• feat(noderesource): lock seid invocation on HOME env var #234 cross-review (security-specialist) — surfaced this
• sei-protocol/seictl — sidecar snapshot upload implementation lives there