Add startup filters for current repo and branch

[jongio]

What

Adds startup filter flags so dispatch can open straight into the sessions for a repo, branch, or folder instead of the global list.

• --current reads the git repository and branch from the working directory (or from --cwd <path> when given).
• --cwd <path> filters to sessions under a folder and doubles as the base directory for --current.
• --repo <name> filters to a repository in owner/repo form.
• --branch <name> filters to a branch.
• --query <text> pre-fills the search box with free text.

Flags can be combined, for example dispatch --repo owner/repo --branch main --query auth.

Why

Closes #177. Dispatch is most useful when opened from inside a project, but it starts at the global session list. Binding dispatch --current to a shell alias jumps to the sessions for the repo you are already in and cuts repeated filtering.

How

The filters route through the existing search-token grammar (repo:, branch:, folder: plus free text). handleArgs parses the flags into a startupOptions value whose SeedQuery() renders the token string that seeds the model, so the active filters show up in the search header and drive the first query. Values with spaces are quoted so the tokenizer keeps them intact.

Repository detection reuses the same owner/repo normalization used for reference URLs, so a detected remote matches the form stored on sessions. Explicit --repo and --branch flags take precedence over detected values. Git detection sits behind a seam for testing.

A non-git directory or a missing --cwd path prints a clear error and does not start the TUI.

Testing

• go build ./...
• go test ./... (all packages pass)
• go vet ./...
• golangci-lint run
• New tests cover flag parsing (spaced and inline forms), query combination, --cwd validation, --current detection and precedence, and the non-git and missing-value error paths.

Docs

README gains a "Startup filters" section and the new flags are listed in the CLI flags table, the usage banner, and all four shell completion scripts.