Deprecate branch name prefixes

[skarim]

The stack branch-name prefix (and the coupled --numbered naming) has caused more confusion than it was worth. The behavior was inconsistent — the interactive prompter applied a prefix while explicit names didn't, and passing an already-qualified name double-applied it (gh stack add refactor/foo → feat/refactor/foo, #152). This removes the concept entirely: from local stack state, the prompts, the init/add flags, and everywhere it was referenced.

Behavior

Branch names are now used exactly as given — nothing is ever prepended or transformed.

• Explicit name — used verbatim (slashes included), so gh stack add refactor/foo creates refactor/foo.
• -m without a name — auto-generated from the commit message as MM-DD-slug (e.g. 07-15-add_login). The slug uses underscores as its word separator, preserves any real hyphens from the message, and is capped at 30 characters.
• Neither — prompts for a name (no pre-filled prefix).

Old stack files that still carry prefix/numbered keys keep loading fine — the fields are ignored and dropped on the next save.

Changes

Remove the prefix concept

• internal/stack/stack.go, schema.json — drop the Prefix and Numbered fields from the Stack model and schema.
• cmd/init.go — remove the -p, --prefix and -n, --numbered flags, the numbered-branch path, prefix validation, and the interactive "use as a prefix?" detection.
• cmd/add.go — collapse branch-name resolution to the three cases above and delete applyPrefix.
• cmd/submit.go, cmd/view.go — stop carrying the prefix through the fork-a-merged-stack path and drop the prefix field from gh stack view --json.
• internal/branch/name.go — delete ResolveBranchName, NextNumberedName, and FollowsNumbering.

Clean up branch auto-naming

• internal/branch/name.go — DateSlug now uses an MM-DD prefix (no year); Slugify separates words with underscores (keeping literal hyphens) and truncates to 30 characters.
• cmd/utils.go, internal/config/config.go — simplify the prompt helper to promptInput and drop the now-unused prefill parameter from InputFn.

Update docs

• README.md, docs/src/content/docs/reference/cli.md, docs/src/content/docs/guides/workflows.md, skills/gh-stack/SKILL.md, AGENTS.md — remove the prefix/numbered flags, examples, and the prefix JSON field, and refresh the auto-naming description.

Testing

• Removed the prefix/numbered tests and added coverage for verbatim explicit names, -m date-slug generation, and the new MM-DD + underscore + 30-char slug rules.
• go test -race -count=1 ./... and go vet ./... are clean, and the changed files pass gofmt.

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[Copilot]

Pull request overview

Removes stack branch prefixes and numbered naming, making branch names verbatim and simplifying automatic naming.

Changes:

• Removes prefix/numbered state, flags, and JSON output.
• Generates MM-DD-slug names with underscore separators.
• Updates prompts, tests, documentation, and agent guidance.

Show a summary per file

File	Description
AGENTS.md	Updates branch utility documentation.
README.md	Documents prefix-free workflows.
cmd/add.go	Simplifies branch-name resolution.
cmd/add_test.go	Tests verbatim and automatic names.
cmd/init.go	Removes prefix and numbered initialization.
cmd/init_test.go	Updates initialization coverage.
cmd/submit.go	Stops copying removed naming state.
cmd/utils.go	Simplifies text prompting.
cmd/view.go	Removes prefix from JSON output.
cmd/view_test.go	Updates JSON output tests.
docs/src/content/docs/guides/workflows.md	Revises abbreviated workflows.
docs/src/content/docs/reference/cli.md	Updates CLI reference.
internal/branch/name.go	Revises slug and date naming.
internal/branch/name_test.go	Tests new slug rules.
internal/config/config.go	Simplifies the input test hook.
internal/stack/schema.json	Removes obsolete schema properties.
internal/stack/stack.go	Removes obsolete stack fields.
internal/stack/stack_test.go	Updates persistence tests.
skills/gh-stack/SKILL.md	Updates agent workflow guidance.

Review details

• Files reviewed: 19/19 changed files
• Comments generated: 1
• Review effort level: Medium

[talum]

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