fix(bin): normalize relative durable paths

[sparkus]

Resolve relative FM_HOME, FM_DATA_OVERRIDE, and FM_STATE_OVERRIDE inputs to absolute paths before deriving durable or cross-process paths, and refuse with the offending variable named when an input cannot be resolved.

Why

A secondmate charter is durable: it is generated in the parent home, copied into the secondmate home as data/charter.md, and read there for the life of that secondmate. It embeds the literal path the secondmate must append its replies to. When FM_HOME was relative, that path was written relative, the secondmate resolved it against its own working directory, and its replies went somewhere the parent never reads — silently. Nothing errored; the reply simply went nowhere.

FM_ROOT was already normalized via cd … && pwd, but a caller-supplied FM_HOME was used verbatim.

What changed

• bin/fm-brief.sh — normalize before deriving DATA/STATE; absolute inputs early-return unchanged so existing output is byte-identical and symlinks are preserved; unresolvable inputs refuse rather than write a bad path into a file that outlives the mistake.
• bin/fm-spawn.sh, bin/fm-afk-launch.sh, bin/fm-bootstrap.sh — same treatment at the equivalent cross-process handoffs, where a later process would otherwise reinterpret a relative path from a different working directory.
• Regression coverage across the affected scripts, including the relative-input reproduction.

Transient relative-home consumers were surveyed and deliberately left unchanged.

Validation

Local pipeline passed with no findings across review, test, document and lint.

[sparkus]

Closing: I raised this by hand, which the repository's own contribution gate correctly rejects — PRs must come through the no-mistakes pipeline so the deterministic Pipeline section is present and the required review/test/lint steps are attested. The underlying change on fm/fix-secondmate-relative-status-path is unaffected and will be re-raised through the pipeline.