fix(security): tighten conversation-history file perms to 0600

[rafeegnash]

Summary

Six of the ten provider conversation-history modules were writing the Q&A log world-readable (0644). The files routinely contain account IDs, ARNs, IPs, role names, and — for iam — entire policy fragments copied verbatim into the LLM answer.

Approach

• New `internal/secfile` package with three primitives:
  • `EnsurePrivateDir(dir)` — `MkdirAll 0o700` then `Chmod 0o700` (idempotent tighten of pre-existing loose dirs)
  • `WritePrivate(path, data)` — `WriteFile 0o600` then `Chmod 0o600` (defends against `WriteFile` not re-applying mode when the file pre-exists)
  • `ReadPrivate(path)` — open → fd-based `Chmod` → `ReadAll` (no TOCTOU window between read and repair)
• All ten conversation modules routed through `secfile` — chmod-on-load auto-repairs files installed by older 0644 binaries, so users don't need to `chmod` anything manually.
• AST-based drift guard (`internal/secfile/drift_test.go`): walks the repo, finds every `conversation.go`, and fails the build if any of them calls `os.WriteFile` / `os.ReadFile` / `os.MkdirAll` directly. The test caught one provider missed by the original audit — `internal/verda/conversation.go` — which is now also patched.
• End-to-end assertion in `internal/flyio/conversation_test.go::TestSaveLoadRoundTrip` that the on-disk file mode is `0600` and dir is `0700`.

Files touched

File	Was	Now
`internal/cloudflare/conversation.go`	0644	secfile
`internal/k8s/conversation.go`	0644	secfile
`internal/iam/conversation.go`	0644	secfile
`internal/flyio/conversation.go`	0644	secfile
`internal/railway/conversation.go`	0644	secfile
`internal/vercel/conversation.go`	0644	secfile
`internal/sentry/conversation.go`	0o600 / 0o755 dir	secfile
`internal/linear/conversation.go`	0o600 / 0o755 dir	secfile
`internal/notion/conversation.go`	0o600 / 0o755 dir	secfile
`internal/verda/conversation.go`	0o755 dir, read-only	secfile

Notes for reviewers

• Windows: `Chmod` is short-circuited via `runtime.GOOS == "windows"` because Unix mode bits aren't meaningful there. Default ACLs already create owner-only.
• Out of scope: `internal/convhistory` extraction (feat(k8s): add k8s ask command for natural language cluster queries #25) and the path-traversal in `sanitizeFilename` (feat: terraform support #23). Both are intentionally deferred so this hotfix stays narrow.
• Sibling bug: `clanker-cloud`'s `backend/internal/{vercel,flyio,railway}/conversation.go` are copy-paste duplicates that also write `0644` — will file a separate issue against that repo after this lands.

Test plan

• `go build ./...` clean
• `go vet ./...` clean
• `go test -race -count=1 ./...` — all packages pass
• `secfile` unit tests: new file is 0600, existing loose file tightened to 0600, dir tightened to 0700, ReadPrivate repairs perms
• `flyio` round-trip test: end-to-end file-on-disk is 0600, dir is 0700
• Drift test fails the build if any `conversation.go` regresses to `os.WriteFile` / `os.ReadFile` / `os.MkdirAll`

Closes #22