fix(do): accept free-form /do <goal>, infer project from conversation

[joedanz]

Summary

The /do command previously rejected any args that didn't match the strict work on <goal> in <project> grammar — so /do fix the readme typo printed Could not parse \/do` arguments. But the conversation already carries a snapshot project_id, so the explicit in ` is only needed when overriding it.

This change adds a free-form fallback: when the strict parser fails, fall through to parseRoutedMessage (the same parser the routed-channel chat path uses) and use the snapshot project.

Behavior

Input	Before	After
/do	resume-from-brainstorm	unchanged
/do work on X in mission	parse + override snapshot	unchanged
/do work on X in nonexistent	hard-reject (E9)	unchanged
/do fix the readme typo	"Could not parse" error	snapshot project + free-form goal
/do fix it, branch off feat/x	"Could not parse" error	snapshot project + branchRef extracted
free-form, no snapshot	n/a	REPLY_DO_NO_PROJECT

The strict parse succeeds only on the work on … in … shape, so all explicit-override paths are byte-identical.

Test plan

• Vitest: 11/11 in do-command.test.ts (8 existing + 3 new — free-form goal, branch tail, no-snapshot)
• Vitest: 78/78 in packages/core/src/messaging
• pnpm build clean
• pnpm typecheck clean
• biome check clean on all touched files
• Manual: send /do fix the panic in a Discord DM tied to a project — should promote to a task using the snapshot project