feat(brief): require crewmates to read project instructions first

[jericho1050]

Problem

bin/fm-brief.sh mentions AGENTS.md/CLAUDE.md eleven times, and every one of them is about writing project memory at the end of a task.
Nothing in the scaffold tells a worker to read the project's own instructions before starting.

That gap has already cost review cycles: shipped code violated documented project conventions because the worker never opened the project's CLAUDE.md.
Those files carry rules no diff teaches - banned commands, required tooling, logging and redaction rules that exist to keep credentials and PII out of logs.
Until now the instruction only reached a worker when firstmate remembered to type it into a brief by hand.

Change

The same short imperative block is added to both setup sections, so it is part of the scaffold contract rather than per-brief discipline:

• Ship briefs get it after the worktree-isolation assertion and before the branch step, phrased "Before your first edit", routing conflicts to the status file.
• Scout briefs get it at the end of the setup section, phrased "Before your first edit or command", routing conflicts to the report, since a scout has no status-file decision loop in the same way.

Both keep the clause that no task is too small to read them, which is the point of the change: the failure mode is a worker deciding a one-line fix does not warrant it.

The existing "# Project memory" section is untouched - it is about writing project memory and is correct as it stands.

Verification

• bash -n bin/fm-brief.sh passes.
• tests/fm-brief.test.sh passes in full (all cases ok).
• Scaffolded throwaway briefs for all three ship modes plus a scout into a temporary data/state root, then diffed each against a brief generated by the pre-change script. The only differences are the new block and the task-id substitutions: the worktree-isolation assertion, the rules block, the project-memory section, and every delivery-mode definition of done render byte-identical.
• Confirmed the rendered output keeps its backticks and leaks no literal backslashes, so the heredoc escaping survived.
• ShellCheck is not installed in this environment - bin/fm-lint.sh reports "ShellCheck not found; install ShellCheck 0.11.0 for CI parity" and exits without linting. That check is unverified locally and is left to CI; the change is prose inside an existing heredoc and adds no shell syntax.

Scope note

The task was scoped to bin/fm-brief.sh only, so no test was added to tests/fm-brief.test.sh asserting the new block.
Worth a follow-up if this instruction should be regression-pinned like the project-memory wording already is.