herdr: optional per-home workspace label (config/herdr-workspace-label)

[thevwick]

Motivation

On a machine running several primary firstmate homes (e.g. one hub per company/context), every primary derives the same constant herdr workspace label (firstmate) in fm_backend_herdr_workspace_label (bin/backends/herdr.sh). All hubs' task tabs therefore co-mingle in one herdr workspace.

This is exactly the mixing problem the P3 workspace-per-home pass already solved for secondmates (2ndmate-<id> labels); docs/herdr-backend.md documents that P2 "assumed a single firstmate instance per herdr session". This change extends the same per-home idea to primaries, opt-in, with zero behavior change for existing single-home setups.

Design

• New OPTIONAL local, gitignored file config/herdr-workspace-label. Its first non-empty line is the workspace label this home's herdr task tabs use.
• Read in fm_backend_herdr_workspace_label via the effective config dir, respecting FM_CONFIG_OVERRIDE / FM_HOME exactly like fm-backend.sh reads config/backend.
• Sanitized: the line is trimmed of surrounding whitespace, then a value that still contains interior whitespace or a colon (the target-string <session>:<pane-id> field separator), or that matches the reserved 2ndmate-* secondmate namespace, is rejected with a stderr warning and falls back to firstmate.
• Secondmate homes ignore the file entirely: their marker-derived 2ndmate-<secondmate-id> labels are already unique.
• Deliberately NOT added to the inheritable-config set (bin/fm-config-inherit-lib.sh): it is per-home by nature, like config/backend.

Zero-default-change guarantee

Absent or empty config/herdr-workspace-label means the constant firstmate, byte-for-byte. A single-home setup, and every existing task's recorded label, is unchanged.

P3 secondmate-label precedent

This extends the existing workspace-per-home mechanism: the P3 pass gave each secondmate home a distinct 2ndmate-<id> workspace to stop tab co-mingling across homes. Primaries had no equivalent knob and all defaulted to firstmate. This adds that knob for primaries, following the same find-before-create adapter conventions.

Changes

• bin/backends/herdr.sh - the label function honors the override for primaries.
• tests/fm-backend-herdr.test.sh - override respected; absent/empty = constant firstmate; secondmate marker still wins and ignores the override; whitespace/colon/reserved labels rejected with fallback.
• docs/herdr-backend.md - "Label derivation" section updated.
• docs/configuration.md - the new file documented alongside the other config/ files.
• AGENTS.md section 2 layout - one-line entry for config/herdr-workspace-label.