Bound REPL history storage and startup memory

[sodejm]

Problem

SecureHistory.load_history_strings() calls readlines() on the entire persisted JSON history before applying its 1,000-entry limit. store_string() only appends, so a long-lived installation accumulates commands without a storage or retention bound. Opening the REPL therefore reads the full file into memory and retains older command text indefinitely. The existing owner-only permissions, symlink refusal, and sensitive-command filtering must be preserved.

Scope

• Read only the most recent retained entries with bounded memory, including when an existing history file is much larger than the configured limit or has an unusually long line.
• Define and enforce a practical on-disk entry/byte retention bound during normal use, with safe migration or compaction of existing files.
• Keep writes and compaction safe against symlinks and partial writes; preserve the existing restrictive file and directory permissions on supported platforms.
• Keep the newest valid, non-sensitive entries in the same order expected by prompt-toolkit. Skip malformed JSON without crashing the REPL.

Acceptance criteria

• With fictional history files substantially larger than the retained limit, startup memory and load time scale with the configured bound rather than total file size.
• Repeated writes keep persistent history within the documented retention bound without losing the newest retained entries.
• Tests cover oversized legacy files, long and malformed lines, interrupted compaction, symlink replacement, permissions, and sensitive-command exclusion.
• No command contents are sent to a provider or emitted to diagnostics as part of this change.

Source

• src/ancestryllm/console/history.py:59-87 — full-file read before applying limit.
• src/ancestryllm/console/history.py:89-114 — append-only persistence.
• Related: Harden REPL history, secrets, and output redaction #58 established history permissions and secret filtering; this issue covers retention and bounded loading.

Intended Feature Release iteration: v0.7 (repository milestone: 0.7.0 Genealogy Workflows).