WEB-4814: merge hooks block instead of overwriting (preserve other tools)

[vigneshsubbiah16]

WEB-4814 (Urgent)

The MDM hook installer overwrote Claude Code's managed-settings hooks key wholesale, clobbering any pre-existing hooks belonging to other tools or prior configuration.

Root cause

Two MDM writers did a blind assignment of the whole hooks block:

Path	Location	Before
Binary (primary)	binary/src/unbound_hook/setup_cmd.py → _write_claude_managed_settings()	settings["hooks"] = _claude_hooks_config()
Python MDM (secondary)	claude-code/hooks/mdm/setup.py → setup_managed_hooks()	settings["hooks"] = hooks_config

The read-before-write already preserved other top-level keys (model, etc.) — only the hooks key was destroyed.

Fix

Replace both assignments with a per-event merge that mirrors the merge pattern already proven in the user-level path (claude-code/hooks/setup.py → configure_claude_settings()):

• Preserves hooks belonging to other tools / hand-authored entries.
• Idempotent: before appending Unbound's current entry for an event, strips any prior Unbound-owned entry. Match is by the hook binary / script path substring (not exact command equality), so a re-run after a binary path/version change replaces the stale entry instead of stacking a duplicate.
• Fail-open (runs as root on customer machines): a hooks value that isn't a dict is coerced to empty rather than crashing the installer; on any JSON parse error the existing read-before-write already leaves the file intact + logs.

Each writer passes its own owner substring: the binary path (HOOK_BINARY) for the binary writer, the managed script_path for the python writer (the latter covers both the Unix "path" and Windows py -3 "path" command forms).

Scope notes

• Touched only _write_claude_managed_settings + the python MDM setup_managed_hooks (plus their tests), to stay non-conflicting with concurrent work on _run_discovery in the same file. No reformatting of unrelated code.
• Codex (_write_codex_managed_settings) has the same wholesale-overwrite shape but is out of scope for this ticket; the hooks.json it writes is Unbound-owned for enterprise installs. Flagging for a follow-up if Codex managed settings can ever be co-authored.
• Cursor enterprise hooks.json left as-is: scoping found that file is Unbound-owned for enterprise installs, and it uses a different schema. Not changed.

Tests (outermost layer)

Binary path — binary/tests/test_setup_migration.py (runs setup_cmd.run([...]) E2E against a sandboxed managed dir):

• foreign PreToolUse hook survives + unrelated top-level key preserved + Unbound entry present exactly once
• re-run over (foreign + prior Unbound) does not duplicate
• a drifted Unbound command (extra trailing args) is replaced, not stacked (path-substring match)
• a non-dict hooks block does not crash; Unbound hooks still written

Python MDM path — claude-code/hooks/test_setup.py (runs setup_managed_hooks() with network download stubbed):

• foreign hook survives, Unbound once, idempotent on re-run
• malformed hooks block does not crash

binary/tests/test_setup_migration.py    19 passed
claude-code/hooks/test_setup.py         20 passed

Pre-existing failures on clean origin/staging (unrelated to this change, verified by stashing): test_hook_cli.py::test_frozen_session_start_makes_no_downloads (4) and test_identity.py::test_keys_limited_to_identity_fields (1).

Refs WEB-4814.

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