Harden hooks/lib/read-session.sh against tab/newline injection in session.json

[5uck1ess]

Problem

hooks/lib/read-session.sh's parse_session_fields serialises 8 session fields as tab-separated values from python3, then splits them back via IFS=\$'\t' read -r. A session.json containing embedded \t or \n characters in workflow, current_step, or other string fields would corrupt the field boundaries, shifting values into the wrong variables.

Today this is low-risk because session.json is written exclusively by the devkit engine (not user input), and the engine doesn't currently put tabs or newlines in those fields. But it's a latent footgun: any future change that lets workflow names or step IDs contain whitespace (or any operator manually editing session.json to recover a stuck workflow) could silently corrupt guard enforcement.

Proposal

Replace the tab-delimited output format with null-byte delimiters, which cannot appear in Python strings and survive all bash string handling cleanly:

sys.stdout.write("\0".join([...]))

On the bash side, read with read -rd '' or mapfile -d '' to split on null bytes.

Alternative: emit the 8 fields as lines of key=value pairs and parse with a loop. More verbose but more robust against schema additions.

Acceptance criteria

• hooks/lib/read-session.sh parses correctly even when workflow/current_step contain tabs, newlines, or shell metacharacters.
• New fixture in hooks/devkit-guard_test.sh seeds a session.json with embedded \t and \n and asserts the guard still enforces correctly.
• No performance regression (the python3 subprocess already dominates latency).

Out of scope

• Replacing python3 with a native Go binary (tracked in Replace python3-based hooks with a native devkit-engine guard subcommand #65 — that's the proper long-term fix and makes this issue moot).

Context

Flagged by Codex during the PR #64 mega-review as a silent-failure vector. Deferred from PR #64 to keep that PR focused. Low urgency because the production writer is the engine itself, which doesn't produce these characters today.

[5uck1ess]

Closing as not-a-problem. (1) The target file hooks/lib/read-session.sh no longer exists — #65 replaced the python3 hook with a native Go binary that reads session.json via encoding/json, which handles embedded tabs/newlines in strings natively. (2) The original threat was theoretical: session.json is written exclusively by the engine, and the protected fields come from YAML step IDs that are already validated to be alphanumeric. No real attacker path ever existed.