fix(security): harden mail handling against header/terminal injection (0.2.5)

[bscott]

Summary

Broad security audit of the codebase surfaced four classes of newly-introduced (concrete) vulnerabilities outside the path-traversal fix in v0.2.3. This PR ships a new internal/safetext package and applies it across SMTP, IMAP, and CLI output to close them.

Severity	Issue	Fix
HIGH	SMTP header injection via Message-ID (copied into In-Reply-To/References) and Subject (ASCII CRLF bypassed RFC 2047 encoding)	All header values go through safetext.SanitizeHeaderValue before reaching the DATA stream. Envelope addresses (MAIL FROM / RCPT TO) are rejected outright if they contain CR/LF.
MEDIUM	InsecureSkipVerify: true for IMAP+SMTP without enforcing the localhost trust assumption — a tampered config or --config redirect would ship the Bridge password to a remote host	Connect() and Send() refuse to dial anything that's not a loopback address.
MEDIUM	mail watch --exec passes the template to sh -c. Currently safe (only numeric SeqNum in {}) but latent RCE if extended to email-derived data	Email metadata exposed via env vars (PM_MSG_SEQ, PM_MSG_UID, PM_MSG_FROM, PM_MSG_SUBJECT) so users have a safe path forward without string substitution.
MEDIUM	ANSI/OSC escape injection — email Subject/From/body/attachment-filename printed verbatim to TTY	All email-derived text-mode prints go through safetext.SanitizeForTerminal. JSON output unchanged.

Bumps version to 0.2.5.

Files changed

• internal/safetext/ — new package with SanitizeHeaderValue and SanitizeForTerminal plus tests
• internal/smtp/client.go — header sanitization in writeMessage, loopback check in Send, CR/LF rejection on envelope addresses
• internal/imap/client.go — loopback check in Connect, header sanitization in buildDraftMessage
• internal/cli/mail.go — terminal sanitization on every email-derived text print sink (list, read, search, watch, thread, attachment download); env-var injection in watch --exec
• internal/cli/cli.go — version bump to 0.2.5
• docs/commands.md — document new env vars for mail watch --exec

Test plan

• go test ./... — full suite passes (new tests: safetext, IMAP loopback rejection, draft header injection; SMTP loopback rejection, header injection, address-list sanitization)
• go vet ./... clean
• gofmt -l clean
• pm-cli version reports 0.2.5
• Manual smoke test against running Proton Bridge (send/reply/forward, watch with --exec)

What this PR explicitly does NOT change

The audit also surfaced four LOW-severity findings (template YAML pre-parse substitution, password-in-Go-string lifetime, --out accepting arbitrary paths in agent contexts, htmlToText href URL filtering). All four are defense-in-depth rather than concrete exploits and are tracked as separate follow-up issues.